I'm sorry, but performance limits do not ensure close racing. As you know, we use restrictions in club racing and the results are mixed at best. While a given group of cars may be capable of similar lap times in testing, doing it in a race is totally different. Invariably, one car is always 'easiest' to drive fast, and everyone chooses to drive it. The result is that you end up with practically a one model grid anyway. Rather pointless, to say the least.
I'm not completely opposed to multi-model racing, but I guarantee that it will never involve an open choice based solely on restrictions. That is the realm of club racing, because neither the BHP/weight limiters or the PP system is good enough to allow that in a championship. Instead there will be a choice from a limited (and fully tested) group of cars.
However, as I've said above, single model championships are the way of things for now. That way everyone is equal, everything is fair, everyone has an equal chance to win, and it comes down to driver ability instead of how good the car is.