-100% cast time[berz, ctm, di, etc] and max aspd should work for that since the after-cast delay of it is limited by your aspd.
Yeah, the skill says that nothing changes his cast time but the rifle from bg cuts it by 25% and other -% cast do the same at pservers[dunno how this is at officials aside the bg weapon that I know that it works there] or at least it used to be like that in the past when I had fun with guns.