My dad just brought home Duracell ProCell AAA batteries. i put them in my friends xmod and it flew. i could hardly control it. these are the best batteries i have ever seen. he only has 4 cells and i still was going SUPER fast. these batteries are 1.5v. if you ever get a chance to buy or use these, DO IT.
Here is a link:
http://www.stagegear.net/duracell_pro_cell.htm