I think it depends on whether you think you'll do more than one 70.3/IM because you are giving up a LOT more speed riding a road bike (doesn't matter what kind of road bike, to an extent) vs. a tri bike (with a good fit). If you think you'll be one-and-done, get the wheels and enjoy them in bike racing. If you think you'll be exploring more triathlon, get the bike. If you are uncertain but leaning towards more tris, get a cheap tri bike, the biggest difference maker is your position on it, compared to riding a road bike.

The difference will always be dependent on each rider but in my case, I have a Cervelo S3 and a Felt IA, both nice bikes in their own right, on a 40k TT in Central park (somewhat undulating, not flat but not "hilly") I had a 7-8 minute difference on a few different tries in various weather conditions. I do think that I have a less aggressive position on the roadie than most bike racers but even if I use the drops, I probably still give up 5-6m in just 40k.

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808 > NYC > PDX > YVR
2024 Races: Taupo

If you want to "buy" speed, there's no question. Get the bike. The difference is bigger than I ever believed before buying a genuine tri bike.

And in spite of what the wheel salespersons might tell us all, aero wheels really don't make that much difference. (Until Roval started selling aero wheels, Specialized's "Win Tunnel" said that aero wheels didn't make ANY significant difference! But since they are now selling aero wheels, they've changed their minds.) Both Helmet and skinsuit / speed suit make a ton more difference than the wheels would make.

You can get most of the benefit of a tribike from your road bike by adding aero bars and adjusting the fit specifically to ride on the aero bars. You'll want to get your seat as far forward as you can as well. However, it's possible that your road bike's geometry just won't work well with aero bars, and you'd obviously be compromising your traditional road bike fit.

Sorry if I wasn't clear above. I do already have clip-on aerobars and a forward seatpost. I've been racing with those for a year now and am comfortable with the fit.

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https://www.strava.com/athletes/5905139

The difference for me (buying a cheap Tri bike) was about 1/2 hr in an IM distance.

The difference in comfort, position and smoother ride was unmistakable.

You would get most of that just buying second hand (old superbikes are fast) and throwing a cover on the back wheel.
It's almost the end of the season, there are some good deals out there including on Slowtwitch.

The 70.3 World Championships was just won on a road bike with clip-on aero bars. (https://www.slowtwitch.com/...v_Iden_AMA_7419.html). In the linked thread he says he had a tri bike (Felt DA I think) but passed on it in favour of the road bike and it obviously didn't hurt him that badly.

The reality is that aerodynamically the differences between an aero road frame and TT frame are negligible. You do have to think about position and cleaning up the front end of the bike but you start talking marginal gains very quickly. The bigger difference between a true TT bike and an aero road bike set up like a TT bike is handling geometry. In most circumstance the TT bike will handle better but the 70.3 championships showed how close the balance is on a technical course.

Arguable you could put wheel upgrades in the same boat. It depends on where you are starting as to what difference you will see. The difference between a decent set to 50mm alloy rims with a carbon fairing and something like the ZIPP 808NSW is a lot of money and a small amount of time. However moving from box section alloys to the 50mm rims will cost less money and net a greater time savings. Similarly going from a box section wheel to a box section wheel with a disc cover represents a much better investment than going from the disc cover to a full disc.

So I see both sides of the argument but if your bike is well setup for triathlon DO NOT expect a TT bike to equate to faster times.