I took a stock AW11 (i.e. 80's) MR2 into the Clubman Cup and used the money I won there to upgrade it. First time I went through I came 2nd, 12th (the straight at Tokyo was bad news), 1st. Second time I was 2nd, 2nd, 1st, and by the third time I had soft sports tyres, less than 1000kg weight and over 200bhp, and won by miles.
I want to use money I win somewhere else to upgrade it fully and take it online, I love that car a lot, but my point is it's fun to enter a car that's massively outgunned in terms of horsepower and see what you can do with it, and only use the money you win with that car to upgrade it. Of course, I could have done 10 laps of Indy and upgraded it all the way before entering it, but it was much more fun using it's superior manoeuvrability to outdrive things like an R32 GT-R and a Celica GT-Four, which aren't exactly dogs in the handling department anyway. Of course, the braindead AI helped.