I ran a Gelcell in my SMD box (very similar to the Losi box) with no problems, other than the weight. Its a heavy battery to lug around with you.
Ive now converted to a pair of 5000mah 2s lipo's and its as light as a.....well, a small ally box
Word of caution though, if you go to gelcell or LiPo, I would consider swapping any connectors you have to Deans or similar. Current draw from the motors when starting a tight engine will melt the Tamiya connectors as quick as a flash.