As I've said before it is impossible to set specific limits unless each engine is dyno tested.
There are a number of ways to limit power to an approximate limit. Engine capacity e.g. F1's 3l limit and the ETCC's 2l limit. Rev limiters, restrictors, or a combination of those mentioned.
As sportscar racing has so many different engine configerations, from low reving US V8s, turbos and high reving F1 type engines, restrictors are the best way to limit engine power.
Each engine configeration has a specific restrictor that will allow the engine to produce an approximate power output, give or take 10-15%. At Le Mans, and I believe the JGTC, the weight of the cars is also taken into consideration. The Vipers GTS-R's for example were pretty heavy and therefore were allowed a slightly bigger engine restrictor to give the car the specified power to weight ratio in GTS. i.e 1100kg =600BHP (approx). If a car weighed 1200kg they would be allowed restrictors that would allow approx. 650BHP.
In the WRC the FIA's theorectical limit for WRC cars is 300BHP, with restrictors set to produce 300BHP approx. This is why the WRC manufactuers always quote official power outputs as 298/299/300BHP. Of course everyone realises the cars produce more like 330-345BHP, but why give away power outputs to your competitors.