I'm between jobs right now, and using that free time to get back into Shift1 racing. It's turning out surprisingly good as I put the time in to get to learn it properly. Previously it was the least favorite of my race titles, but that's no longer the case.

If anyone's still playing this, this is what I did to get very decent racing out of it:

1) Drop in the Sharper Tyre Mod 6.01 by Juls
2) Remove all "Gadget Cars" that are not GT1 and GT3
3) Add the mod that allows to buy and tune the Special Bonus Cars
4) Change the car values so that "proper" GT3 and GT1 cars will race in the same class.
5) Extend Time Trial to 15 Minutes - four sessions of 15 Minutes each make up practice for the race, with setup changes being done between.
6) Race 10 Laps or above in Quick race.

I'm finding that all cars in each class do have enough handling difference to be fun to learn, but yet I can get about the same times in them when driven properly. During the actual race against AI, it's the closest-matched and most challenging racing now. I just spent three laps at Donington in my Porsche trying to get that 'Vette in front, each of us making slight mistakes that evened out. Really hair-raising stuff. If this thing had qualifying it would be the most wonderful GT Racer today.

If anyone wants the files that reduce the car set and put up competitive classes, PM me.