For full IMs I run a round bottle of 750 ml on my p5 on my downtube for my gels.
I consider to swap it to an xlab aero tt,
Although 600 ml is basically too small I could carry the first 4 gels in my BTA water bottle.

I wonder whether the hassle is worth it wattwise. Otherwise stated: Are there any figures showing the aero advantage of an xlab aero tt over a round bottle on the downtube of a p5?

I don't know what figures are available but I would say that if it's worth buying a P5, it's worth NOT mounting a 750ml round bottle on the downtube. Is BTS available or already used for water?

I carry a BTS bottle only for spare parts. I tried to use it for fluid, but the risk of dropping and the hassling to get it in or out (you need a tight fit like the gorilla xt) are too great for me.

Did you see Lucy Charles drop her BTS bottle just past Hawi? I would not say she's clumsier as me.

My two BTA bottles are reserved for plain water (to drink with the gels) and for electrolyte.

I do not allege I've the best system, but it works for me and I try to find out if the difference in aero advantage over a round bottle of an xlab aero tt is as acknowledged as that of an elite crono (which latter I do not want).

I'm sure it works, but then so would riding on the base instead of the extensions. It just doesn't make best use of your options.

While I agree that BTS is not ideal for regular access, it seems feasible for occasional gulps of gel, if that's how you would use your gel.
I use a BTA for drinking and a BTS for refills. It's easy enough with a little practice but yes, you could drop a bottle if you're not cautious and caution takes longer. So I suppose the question is whether it costs you more time to grab and replace a BTS bottle or to add drag with a round bottle, or is an aero down tube bottle the best compromise. I would think the round bottle is almost certainly the worst option. Choice between the other two depends on whether you can re-organise spares storage to free up the BTS without additional drag, whether you use the gel bottle too often for BTS to be practical and time saving, and how well the proposed aero bottle matches the frame. oops, Maybe I'm just re-stating your question and not answering it ;)

I bought my Gen 1 frameset from a World AG Hour Record holder, who'd tested it in the Tunnel. He said seat-tube was faster on the SC. I've used a Vitrue bottle on the S/T a couple of times, but that may well not be the fastest (bottle) option.

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29 years and counting