I've seen a very good engine builder get very high 80's out of a 450cc with the 400cc making high 70's.

My old ZXR400 race bike was full of special Jap parts and made 76hp if I remember correctly. The specs actually quoted 83hp or something, but I think that was measured at the bottom of a bottle of Sake.

A CBR250 is supposed to be making 40 odd horses, and they actually feel quite quick once you get them spinning up, so there's probably no reason why you couldn't get a 400 up to 90hp if you had an endless supply of cash for trick cranks, rods, pistons etc etc, and 20,000 rpm to play with.