I think that track by track balancing is wrong, if your gonna do that you might aswell let people chose their cars track by track. A better option if you wanna make it fair (depending how you look at it) so much would be to do success ballast.
I personally am completly against track by track ballast, it wouldn't work in terms of time used and i wouldnt be happy having a different car to the one i signed up to use for every race.