By going to a geared setup you will minimize the chances of bending the shaft on the motor and it will also allow you to run a more efficient prop.

As Montague said, just the LIPOs alone will increase your run time. You should be saving weight on the pack while increasing the mah.

As a precaution/word of advice, I strongly suggest upgrading the ESC if you go to LIPO and stay with the brushed motor. Get an ESC that has a programable voltage shut off so you don't over discharge the LIPO. I do know Castle makes them (definitely for brushless), however, I don't know who else.