There used to be this addon: http://www.facepunch.com/threads/585848-gmsv_vphysics-Physics-Performance-and-Speed-Settings?highlight=unlimited+speed that let you change, amoung other things, maximum prop speed, and the modules for it were added to gmod by garry quite a long time ago.
And because of that the creator of that addon released a simple script that would add those commands back into gmod without needing to install a whole addon, since the modules were there but normaly only used in luascripting.
And that script still works except for one thing: changeing sv_maxvelocity does nothing at all. So could someone please take a quick look at it, and see if its possible to add a working sv_maxvelocity?
Also, a link to the wiki page for the modules: http://wiki.garrysmod.com/?title=Physenv