I think 300whp for 10+ years is viable. It only takes a tune which will actually have the car running better than stock.
You can probably bump 400whp to 5 years. Assuming the engine isnt built at all I give someone running this whp 5 years before a boost spike kills it.
800whp would be fun to experience but not in my car, haha. I would choose the lower whp to maintain good reliability and focus more on weight reduction to make it fast.