IIRC, it is 5% difference between the lowest and the highest tiers. What I know for sure is that the scaling is only present in career. So if you drive a single race, 100% AI means 100% AI, no matter what car. Same will probably go for the custom season, once that feature (that imho should be in, in the first place!) gets added.
Thanks for clarifying on that!
And
@kkdrummer: Besides what Bealdor said, not every design decision was discussed at length with the community in a 50+ pages thread. Of course not, otherwise the game would still be nowhere near finished. This is something that the members just didnt realize until it was too late.