Sorry but I can't agree that driver ability is the ONLY way to do it... Might work for the BDC, but splitting by car type works in EVERY other form of motorsport I can think of, and every form of motorsport has a range of abilities.
Personally I have only been into drifting for a few months, I have 2.5 days track experience & although in the process of building something more potent, currently share a car with a mate that is running 135bhp in a 1400 kilo shell... so I'd be in the lower class however you measure it... however, I would much rather finish behind a far better driver in a similar car than a similar standard driver in far better car.