Pretty much agree with all of that, that speed limits are partly there as an attempt to "catch all" and cover the lowest common denominator.
Speeding is a little like smoking; many people do it even though there are potential negative consequences, both for the individual and those around them. In short it's inappropriate speed, not inappropriate speeding, that's the greater threat, though seeing as individuals will have differing views as to what inappropriate speed is you have the situation we have now whereby maximum limits are set. It doesn't mean that driving at 100mph by an alert, competent driver in a suitable vehicle in safe road conditions isn't safe, however that's where the grey area creeps in - how do you define what an alert, competent driver in a suitable vehicle is?
For the record, I'm partial to the odd smoke and I also, on occasion, exceed the speed limits.