What effect will voltage difference have?
 

I have an Axial Yeti XL and am currently running two 7.4v 2S 5000mah LIPOS. I thought that 7.4v was the maximum you could run on 4s but I just saw in the manual that you can run two 11.1v lipos in 4s, what difference would that make, performance or run time? I'm a bit of noob so a little insight would be appreciated, thanks all.

I bet you would see a big performance increase with the 3S packs. Run time is dependent on mAh, so that will be a matter of getting equivalent 5000 mAh packs.

Using (2) 2S LiPo pack is putting out 14.8 volts. Using (2) 3S LiPo packs is putting out 22.2 volts. I couldn't imagine running that monster buggy with 6 cells. The general consensus that I read about powering the XL---sweet spot is using two 2S packs. The power increase from using two 3S packs makes the Yeti XL quite a handful to control.