I also drift many sedans like the quattroporte, series1 and recently the ISF which I assume your talking about. I run my cars at 400bhp and 199mph topspeed everytime and as long as you change gear mid drift its fine. The ISF on the other hand beats them all, it goes through hairpins redlining in 6th.
So my advise is to stick to any 2 or 3 cars at a time and learn what gear to go into each corner with and what gear to be in when you come out.