For a detailed explanation of TT vs road bike with real-world testing, see this previously posted link:

http://www.bikeradar.com/news/articl...-is-aero-19273

Of note is the final comment in the concluding paragraphs:

". The difference between the Specialized Tarmac SL2 (roadie) with clip-ons and the Transition (TT bike) was 18.5W or 19.3W. Finally, the difference between a road frame and one with a set of clip-ons on it was a whopping 29.4 watts. This difference is due to rider position (in the drops vs. in the aerobars)."