The point is that 99% of the time it doesn't have to be regulated, the drivers do it themselves. No-one wants to win just by having more power, I guarantee it.
It's even easier with GT, because there is data logging and it is so easy to tell if someone is not adhering to the rules, 'is the gap getting bigger?', 'was he at full throttle when it was KNOWN that he had a power advantage?'.
The other simple fact is, in real life, it is a REQUIREMENT that the car NEVER straightens up other than for the run-up, so, since the car should ALWAYS be drifting, straight line speed is not even an issue. The same should go for GT really, the judged sections should REQUIRE that the driver NEVER straightens up, therfore it makes no difference who has what power.
Besides which, power is taken into account while judging, this is another reason why it's easier to judge in GT, we have known power figures. In real life, people sometimes 'sandbag' by saying their engine is stock and only makes 300bhp when actually it makes 390 etc etc, this is not an issue in GT.
As far as I can see, it's people trying to 'level the playing field' when in fact all they are doing is making it boring. You are limiting what cars will enter by saying that you have to find whatever car works best with 400bhp (or whatever power is limited to). Simple fact is, in GT, with a circa 400bhp limit, that car is the FD RX7, so what is the next thing? Ban RX7's because they develope so much more grip than most other cars? Why should I be disadvantaged by wanting to compete in my R32 (or whatever I choose), when it is blatantly obvious that an RX will piss all over a similar power R32.
I've read all the reasons people have given, and it still seems to me like the people restricting power have no idea how power actually affects a the performance of a drift car and they are trying to level a playing field in a way that is, in fact, totally ineffective, and actually just makes the playing field more uneven as it is basically saying 'you have to drive 'x' car to do well'.
Rubbish, the fact is, most 450bhp cars will build more speed between corners than, for instance, my Viper, because I keep the wheels spinning whereas they grip and accelerate,building more speed.
You don't need macros, it's easy. Besides which, in real life, if the slower car backs off the throttle (or the single car if it is during qualifying) is required to be FULL THROTTLE on ALL straights right up to the point of initiation. If he lifts, it is considered a MASSIVE loss of points as he is not going as fast as his car is CAPABLE of going.
Re: Braking, again, touch the breaks in a straight line = MASSIVE fail.
That was down to grip, I'd have done the same thing on stock power. The fact is POWER made no difference. Besides which, only time anyone can really catch me is if they're in an RX

LOL I know this sounds REALLY arrogant, but that's life, I've always been a very fast drifter (in real life) due to my race-car experience, so even in identical cars I suspect you'd struggle to keep up if I was leading and wanted to pull a gap.
Feel free.