Medmon.. your mileage may vary.. but using that environment sphere(base studio install in the props group of smart content) as a light has tripled to quadrupled my render speeds.  It seems to really do the trick if you're just using ibl anyway.  When I use mesh lights the max speed I can get out of this 4yr old puter is about 30k s/s; so, I use any trick I can to squeeze more speed until I build a new box in october.