Main reason for the ~400fps limit is because people skirmish with them (shoot each other in wargame scenareos); if you have a higher speed, they become classed as firearms (in the Uk atleast), as they are full auto capable in many cases, plus anything much over 1J of power can be 'lethal' if you are shot in the face (eye). If you just want a high powered fun gun/ sniper rifle, thats what proper airguns are for really; Im rockin a RWS series 500+, .177 calibre monster:
and my target airpistol, a Weihrauch HW75:
