Not really; we can't have a high-cap battery because the N800 wasn't popular enough for Mugen to make one. While it might look a little odd, there's no reason an extended back couldn't be made to suit the N800. FWIW, Mugen does make an N800-compatible extended battery, but it comes with a useless E61/E62 back. Indispensable, as long as you're willing to trust two strips of scotch tape as your only insurance against hard power-cuts...