Here is how Performance Difference Adjustment works:
For every 1PP you are below the limit, you get a bonus after your login bonus equivalent to 6.6% of the race's base prize money/XP for your position.
I'll assume it remains the same for positions other than winning, but I haven't not won yet to report back. I've not worked out the XP because I'm 40/40 and don't presently care, but I'd guess it's similar (Edit: Yes, XP is identical). I've tested multiple cars in multiple races at all the tracks and, for winning at least, that's exactly how it works.