There are sliders for ballast and restrictor in the "drive" section of Content Manager so presumably hidden somewhere in the server settings for multi.
Unfortunately they're not as effective as one might imagine. With 100KGs of ballast and 50% restrictor for the M235i I am still a second quicker than the Skoda at Silverstone National. We could keep piling on the weight and restriction but it starts getting a bit silly. Also had a quick drive in the Audi S1 and it's 2 seconds slower than the Skoda.
I'll have a look at some other Kunos cars later to see if there's anything close to the Skoda but I'm probably looking at road cars
How about old Audi Quattro or M3...or just let Goldtop free choose.