From my experiences, 400bhp per ton seems to be a good level that is properly entertaining for a long time, on most roads its gonna be hard to get bored of that, but you will on longer, straighter, safer ones.

TBH, i reckon even at 500bhp per ton itd take a lot, even motorway driving, to think you fancied more power as you was bored now.

Most ive driven is about 900bhp per ton, and that was properly frightening, so perfect. That id expect would be hard to get bored of in any situation.

Its mostly about where you are driving though, as obviously wider, straighter, quieter roads, at higher average speeds, means you will get bored easier of a set bhp/ton, as itd have less noticable effect.
My comments above are based on all-round driving, as sub-100mph on single lane road, even 400bhp per ton is prob enough for ever I reckon, just sometimes need more on easier roads.