I train in the TT position most of the time and set my avatar up with the P5. For the times I am doing hard intervals I sometimes switch to the S5. What I’ve found is that there is very little speed advantage using the P5 over the S5 compared to what you would see in the real world, even allowing for the drafting benefit of the S5.
I expect to see a CdA differential (un-drafted) of around 0.0400 (0.2000 for the P5 and 0.2400 for the S5). these numbers are what I see in the real world at 40kph. My TT bike is a 51 P3C with Super9 and 808, and my road bike is a 54 P3C with aero road bars and ENVE 7.8, so very similar to my virtual bikes.
Given that you can’t race on a TT bike (as far as I know, except in TTs) I’d like to see a more realistic speed on the virtual TT bike in Zwift for my 200 Watts 
The only reason I can see for not doing this is that some might chose a TT bike and then ride in a road position and thereby gain more XP per watt when free riding. I’m actually better in the TT position than sitting up as I’ve been training in the TT position for ten years.
