I'm running mine at the 525 and it's excellent. Yes it requires a small amount of de-tuning but most race cars carry some kind of power restrictors so i think it's spot on!
As for the PP and BHP limits, i suggest just keeping it at the 525 limit. To be honest their is absolutley no way beyond trust that the BHP limit can be policed. Some will run more bhp, some will run less - however, weight will come into play there and could make for some very interesting races as a car with a bit more mass can suit some circuits better, especialy when that ballast can be moved in the chassis.