My 2p. FWIW.
My fastest tt's I've done on a £600 alu road bike with clip on tri bars (Boardman), my fastest bike section on a Oly was a carbon tt bike. I don't think it matters too much till you start to get faster say over 25mph average.(Not me any more).
Plus you battle with comfort on a tt bike. I've done long rides on tt bike in training (100 miler) and found it hurt my shoulders, arms and hands (weight too far forward, bars too low). I've also done long rides on a carbon road bike (Boardman again - 130 miles) and found it much more comfortable for no difference in average speed.
I'm lucky enough to have a choice between a carbon road bike and a carbon tt bike for this weeks Ironman, I'm going for the road bike for comfort, it's lighter and it descends better . If I average over 19mph ( which I doubt I'll do ) over 112 miles I'll be very pleased.
Ergonomics vs Aerodynamics, its a trade off. But you'll get a bigger range of use out of a road bike. (Randonee's etc).
BTW I've waited for 12 months for Planet-X to tell me what size bike I need, two visits to the shop, 4 phone calls and many emails - nothing.