Yes it's true that 1600 is the largest receiver pack made with 2/3A cells, but you can also fit a 5 cell 3+2 hump made with AA cells which are available at any hobby site pretty much, that can give to 2500-2800mah, another alternative which you probably aren't after would be to go with a 2000mah 2S Lipo and regulator, that will give you also about 2500mah (because of the lipos properties being regulated down) of battery power. Check out A Main Hobbies battery selection.