Leadsled,
The answer is no, because I know where the "250kt" rule came from ---- the early days of jets, when the majority of airline traffic in the US were big pistons. The limit was imposed to reduce ATC problems with the huge speed disparity between the new fangled jets and 049/749/1049/DC-4,6,7/Convair 240/340/Martin202 etc and B707/DC-8.
Wrong. It was exactly as ARFOR suggested: introduced by the FAA to help See and Avoid after the midair between DC-9 N1063T and a Beech Baron at 4500ft.
Mixing jets and bugsmashers, relying on See and Avoid is just plain DUMB.