I may be wrong, but in term of absolute numbers, wrc engines don't seem too high tech to me. F1 Honda '88 engine with same 2,5 bar boost reved to more than 12000... I know F1 is not relly, but it was 30y ago.

I can imagine that it is tricky to achieve optimal effect with limited amount of air (optimal combustion), but it should not be problem to make it mechanicly reliable...