I've no idea how much power my slightly modded uk classic has, below 300 anyway, but having been on plenty spirited runs following scoobs of all ages with loads more power than me, claimed more anyway . i've never had any problems keeping up, granted, as stated above they'll pull away a bit on the straights, but not enough to really make any difference. Also, i can be fairly confident in the fact that i'm not going to blow out a piston or shred my gearbox.
I'm not saying people shouldn't have huge horsepower cars, there's some really impressive motors in here but on the real roads it doesn't make much odds.