BACK ON TOPIC....
The Speed 12 and Buick have insane amounts of power and torque while being stable. From what I understand, GT5 calculates drifting points by factoring in speed, angle, what gear you're in, and possibly racing line as mentioned previously in this thread. So what the Buick and Speed 12 are good at doing is spinning the wheels in 6th gear, and I'm also thinking that there's something about their suspension geometry that allows for increased steering lock, hence good angle. The tyres typically run on these cars are staggered; I think it's grippy ones on the front and low grip tyres on the back, but I might be mistaken, they could be the other way around.