My usual response to the question is along the lines of:
'The national speed limit in uncontrolled airspace is 250 kts below FL100. I cannot clear you to exceed that speed, and you do so at your discretion'.
It's a bit like road speed limits: If you break them and get caught then you have only yourself to blame.
Before anyone jumps in and tries to protect the puddlejumpers (such as me on occasion) who would not relish and A300 doing 300 kts across their nose (or worse), I would add that I am only asked this at night when there is b****r all else showing on the radar in that area.