I've ran my 4s zippy flightmax lifepo4 battery several times in my slash & so far no problems & it runs great! it's still geared a little too high (21/86) & it gets a little warm after 15 or 20 minutes but not too bad & the temp outside was well over 100 degrees. I'm gonna put about an 18 tooth pinion on it & if it still gets too warm I'll put a heatsink with a fan on the motor.
http://youtu.be/Q42xVonAVys