3cell lipo is not really practical unless you're running brushless. Seems a lot of people use custom electronics to run extreme voltages, but I think they should look to replacing the stock electronics with some hobby grade stuff first. If you've ever seen a 1/12 run on 4 cells, you'd understand why I say that
10C is pretty solid, but newer cells (especially for the smaller mAh like you're looking at) are higher than that.
I've heard great things about
http://maxamps.com but I haven't used them myself. Their 380 cell is rated to ~20C or 8A burst (34mm X 50mm X 2mm per cell), and the 900 is ~15C and 13A burst (34mm X 50mm X 4mm per cell).
Any and all of these should blow away the basic li-ion solutions that EVERYONE sells for xmods. You might try batteryspace.com if you want cylindrical li-ion cells, they have a few options, one is ~18x50mm and discharges 3.5A with a capacity of 1400mAh or so.