Tug
I would imagine that the 250kts limit works well in practice as a number because most jets can fly clean at that speed??
I agree with your view about the inherent dangers of see and be seen - I had a Challenger appear within 1/4 mile out of nowhere at apparently "high speed" in class G over Essex a few years ago and it gave me quite a shock - and its sobering to reflect that it was probably observing the 250Kt limit too.
Can't help but think that having some people in the same area talking to ATC, others to AFIs, yet more to A/G and some not talking at all is not the ideal way for the left and right hands to coordinate.
I prefer a system such as SOCAL approach in LA, where everyone speaks on a common frequency (assuming they speak). I will admit that they also have the benefit of a superb radar service (aka Class C or Flight Following in old money.)
The shrinking of the LARS service is also very unfortunate - it was a significant aid to air safety in my opinion.
As to whether gliders etc should have transponders, I'll leave others to respond.
However, to return to the original point of the thread, if there are pilots out there speeding, they should not. It is illegal.