I have been doing triathlons for a couple of years now--sprints, olympic distance and even a 1/2 ironman, all on a road bike. I never expect to compete for top spots (unless I am suddenly possessed by the spirit of Michael Phelps), but I wouldn't mind getting faster where I can. What is the opinion of the experts on how much a tri-bike would improve my time? For instance, I did the bike portion of the 1/2 in 3 hrs 15 minutes. If I had a tri-bike, would I expect to automatically be 15 minutes faster?

Thanks for any input!