While low base bars and pedestaled elbow pads may be the fastest/best solution for a pro time trial, I think most triathletes would be better served having their bull horn position mimic their hoods position on a road bike. It would be better for climbing hills, recovery rides, and riding in a group if you have electronic shifting.
The two bikes pictured below share the same saddle setback and pad stack and reach. The drop from saddle to pads is 9.5 cm. The saddle setback is 6.5 cm, so pretty far back considering both are short saddles. The Cervélo road bike frame is fitted with an 80mm -17 degree stem and Pro Missile integrated aerobar. The Scott has a Specialized integrated aerobar and the equivalent of a 60mm stem. The elbow pads on the Cervélo literally sit on the base bar. There are 4cm of spacers under the pads on the Scott. The chief difference is that the bull horn position on the Cervélo road bike TT conversion is about 4cm higher than on the Scott tri bike and is close to my road hoods position. The Cervélo also weighs 2.5 lbs less.
I have not tried to measure any aero deficit the road bike TT conversion may have compared to the tri bike, but I doubt it would be much. Tom Zirbel won the 2013 US Pro Time Trial title on an Orbea road frame with a HED Corsair cockpit and downward angled stem. While the road bike conversion may not be as stable as a tri bike, I have no difficulty riding a 60mm deep front wheel on the Cervélo.
I understand that those that want to ride a more aggressive position need to be able to get the front end lower and the saddle further forward, precluding using a road bike with a TT cockpit unless a downward angled stem and forward seat post are employed (which could negatively effect handling). But perusing most of the tri bikes in the transition area at a local race, most are not set up as aggressively as these two bikes. They are using high stack aero bars (at least 6 cm of pad stack) and some combination of spacers under the stem or upward angled stems to get their positions. Unless they have super bikes like the Trek Speed Concept, they are ruining the aerodynamics of their bikes with stem spacers and upward angled stems and still have a bull horn position that is much lower (and further away) than their hoods position would be on their road bike. If they want to preserve aerodynamics (no stem spacers, -17 degree stem), they end up with a bull horn position that is so low it is awkward for almost every situation except sprinting out of the saddle.
For athletes without super aggressive positions, why not offer an aero road frame that has about 4cm more stack than the average tri frame and 2cm less reach? One could then spec a low stack aerobar and zero setback post. The new Profile Design subsonic 35a aerobar and WING/10a base bar would be a good choice for the cockpit. The fit of the bike could be fine tuned using stems of varying length and angle, hopefully no higher than negative 6 degrees, and maybe up to 2cm of aero stem spacers. This would give the athlete a fast TT machine that also works decently as a road bike. The triathletes that want to ride an aggressive position can buy the pro TT bike or the more integrated triathlon model. Even triathletes with an aggressive position might be best served using an aerobar with very low pad stack or an upwardly angled base bar in order to get their bull horn position higher.
It is worth noting that both bikes below are tall and short for their respective types. The road bike has a stack of 58 cm and reach of 387 in size 56. The Scott tri bike has a stack of 54 and reach of 395 for size 54. Athletes that do not share my long leg/short torso morphology could easily choose a road bike frame with 2cm less stack and achieve similar drop to the elbow pads. If the tri bike were a Cervélo P2 in size 54, not only would my bull horns be 2cm lower (if I wanted to use a -17 degree stem), keeping the reach to the bull horn position the same would require a base bar with 2cm less reach (Vuka Bull?) or 40 mm stem, which I doubt would be advisable.
The two bikes pictured below share the same saddle setback and pad stack and reach. The drop from saddle to pads is 9.5 cm. The saddle setback is 6.5 cm, so pretty far back considering both are short saddles. The Cervélo road bike frame is fitted with an 80mm -17 degree stem and Pro Missile integrated aerobar. The Scott has a Specialized integrated aerobar and the equivalent of a 60mm stem. The elbow pads on the Cervélo literally sit on the base bar. There are 4cm of spacers under the pads on the Scott. The chief difference is that the bull horn position on the Cervélo road bike TT conversion is about 4cm higher than on the Scott tri bike and is close to my road hoods position. The Cervélo also weighs 2.5 lbs less.
I have not tried to measure any aero deficit the road bike TT conversion may have compared to the tri bike, but I doubt it would be much. Tom Zirbel won the 2013 US Pro Time Trial title on an Orbea road frame with a HED Corsair cockpit and downward angled stem. While the road bike conversion may not be as stable as a tri bike, I have no difficulty riding a 60mm deep front wheel on the Cervélo.
I understand that those that want to ride a more aggressive position need to be able to get the front end lower and the saddle further forward, precluding using a road bike with a TT cockpit unless a downward angled stem and forward seat post are employed (which could negatively effect handling). But perusing most of the tri bikes in the transition area at a local race, most are not set up as aggressively as these two bikes. They are using high stack aero bars (at least 6 cm of pad stack) and some combination of spacers under the stem or upward angled stems to get their positions. Unless they have super bikes like the Trek Speed Concept, they are ruining the aerodynamics of their bikes with stem spacers and upward angled stems and still have a bull horn position that is much lower (and further away) than their hoods position would be on their road bike. If they want to preserve aerodynamics (no stem spacers, -17 degree stem), they end up with a bull horn position that is so low it is awkward for almost every situation except sprinting out of the saddle.
For athletes without super aggressive positions, why not offer an aero road frame that has about 4cm more stack than the average tri frame and 2cm less reach? One could then spec a low stack aerobar and zero setback post. The new Profile Design subsonic 35a aerobar and WING/10a base bar would be a good choice for the cockpit. The fit of the bike could be fine tuned using stems of varying length and angle, hopefully no higher than negative 6 degrees, and maybe up to 2cm of aero stem spacers. This would give the athlete a fast TT machine that also works decently as a road bike. The triathletes that want to ride an aggressive position can buy the pro TT bike or the more integrated triathlon model. Even triathletes with an aggressive position might be best served using an aerobar with very low pad stack or an upwardly angled base bar in order to get their bull horn position higher.
It is worth noting that both bikes below are tall and short for their respective types. The road bike has a stack of 58 cm and reach of 387 in size 56. The Scott tri bike has a stack of 54 and reach of 395 for size 54. Athletes that do not share my long leg/short torso morphology could easily choose a road bike frame with 2cm less stack and achieve similar drop to the elbow pads. If the tri bike were a Cervélo P2 in size 54, not only would my bull horns be 2cm lower (if I wanted to use a -17 degree stem), keeping the reach to the bull horn position the same would require a base bar with 2cm less reach (Vuka Bull?) or 40 mm stem, which I doubt would be advisable.