OK, this is very strange. First of all, I've done a fair amount of modding with F1C and early days of rFactor, and I know the drill when changing parameters and setup details; so I think it's unlikely I've pulled any noob mistakes - restarting the application, reloading mods, etc. etc. I'm running v946, which seems to work just fine with older and newer content to date.
I played around for most of an evening with the IZOD 2010 setups and while I was able to make changes to the top speed, I never got a top speed much over 200. I even pilfered a setup from the rF1 IndyCar mod and it made no difference - the rF1 cars attained full speed but the rF2 cars didn't.
So, thinking I might try to open the car package and look at engine parameters, I shut everything down (again) and installed the latest dev tools. Then I started the mod to take some notes on the likely suspects - RPM, drag, etc. When I went to the garage, many of the setup parameters looked like they had changed. I re-loaded my test setup to put them all into a known state, and it did - I checked my earlier notes to make sure none of the important values had been corrupted. Then, puzzled, I took the car out onto the Indy oval and lo! The car now would effortlessly accelerate past 220 MPH with the same exact settings which were giving me problems ten minutes earlier. I loaded an rF1-created setup for comparison, and it was fine too (though possibly not optimized for rF2, it still worked).
I have no idea how installing dev tools could affect mod performance in such an odd way. But after the install, something indeed changed.