I went from a 1989 Ironman Expert, all Suntour GPX to a 2007 Kestrel Talon (all carbon, 9-sp DA) and the difference was pretty much moot.
My PR for the bike leg is still the day on the Ironman. I never missed a shift, kept momentum in the corners, never came off the drops.

The difference was so neglible, the transition times were the only variable. I had a better transition on the Ironman, as I ran the 5K (on a zoysia golf course) barefoot.... The race didn't measure transition times, so the 24.5 on the IM beat the 24.3 on the Kestrel.