Many people have done 5- and 6-cell mods to their xmods. I would be cautious about going with more than that (even if you could fit the batteries under the body) without replacing the stock electroncs.
Most after-market ESCs can handle much higher voltages, but I doubt the stock PCB would hold up very well to anything beyond 6 cells.
Of course, you're welcome to hook your xmod up to a car battery and tell us how it performs

Might want to use a long pair of wires though...