Does it have too much power? No (unless you think a modern 450 has too much power). It is rather slower revving than a modern bike, makes it less aggressive. 600s can be pigs to start, odd because the 501s are a doddle. As the earlier post says, they vibrate a bit.
I raced a 600 in the mid 1990s and it was the best 4 stroke out there.That was 15 years ago though and the competition was ATK's, LC4 KTMs and 610 Huskys. All big heavy bikes. Against modern bikes and jump-filled tracks you will struggle - but if it is a fun ride, rather than wins, you want then who cares? You will have a ball and sure to get plenty of paddock admiration. (Get a KTM 525 for the wins).
Best money you can spend is with a good suspension shop.