There's as many answers to this question as there are people on this site. What's not enough for Peter may be way too much for Paul. With no standard thresholds of skill, bravery, ability, reflexes, stupidity, testosterone or taste, it's sheer madness to expect a correct answer or any sort of consensus. Imposing your standard or threshold on someone else and expecting them to accept it is even crazier. At best, we can share our own individual takes on the issue and enjoy hearing the varying perspectives of others.
Me personally, I've owned and driven a few abnormally powerful cars - way more than is required for even spirited driving on the street, and I have never ever felt like saying "this is enough". I hear people saying that they don't like 400 - 500+ HP cars because they feel they can rarely or never experience the full potential in daily driving. If I believed that I need to be able to floor a car in every gear for me to enjoy driving it on the street, then I'd find my thrill in any number of conveyances of basic transportation.
I love being able to tap into awesome, tire shredding power once in a brief while, and feeling the JATO rush in otherwise routine acceleration and passing manoeuvres without necessarily breaking into triple digit speeds. I also love having the power to shoot up into those triple digits quickly and back to normal again when it's safe to do so without excessive risk to (my and others') life, limb and property.
At the same time I fully understand some peoples' love for stirring the gears and running up the revs while flogging the hell out of lower powered, sporting little cars (MG's, Sprites, etc) that barely manage in all their revving and and fury to get out of their own way. I appreciate the automotive enthusiasm, but it's just not my cup of tea, that's all.
