Hmm, there are many variables which affect the performance. Even without restrictor the engines of WRC still have rules for their creation which specify a lot of properties, materials, fuel, reliability etc. I think that the current 1.6 turbo engines could not go into sky-high power even without restrictor. There is relatively low turbo pressure and the maximum rpm are limited by used materials, con-rod length or steel valve springs. I'm not motoring engineer but 450 Hp is sure possible, maybe more.

Otherwise with completely free rules, free fuel and no reliability it is possible to go into completely crazy values. If I'm not mistaken in 1980' the qualification engines of F1 had up to 1400 Hp from 1.5 litres but the engines could last only two or three laps.