I ride with a PowerTap on a regular basis.
At equal power, My P2C gets me about 2 - 2.5 mph faster over my road bike riding in the drops. That's with spoked wheels on both bikes. If I put on the Crosswinds, I can get a 3mph difference. Both are well fit and good quality wheels, hubs, etc... I would attribute 90% of that increased speed to the my more aero position on the Cervelo. You will not get this low and aero by slapping some aero bars on a road bike.
But, if you're going to get a TT bike to go faster, you have to be committed to staying aero the whole ride and it is not fun or comfortable.
I'm pretty average, but my best 40K was 56:56 and I got 3rd in my age group a few years back with 58:31. Both were on the P2C. My best time on a road bike was around 1:05 but I was much younger then too.
Last edited by andr0id; 09-29-14 at 05:34 PM.