It sounds like your motor isn't turning enough KV...i would go back to the lipo with a minumum of 20c..upgrade the brushed motor to a brushless 400-480,and run a 10*6 prop...I use a 25c lipo, 450 brusless exceed rocket motor and 30amp esc with a 10*7 APC prop. after adjusting the diheidral  on the wings abit the plane screams with speed n climbing power..