the xl5 wont handle a 11 turn 540. the stock 12 turn is a 550, which is more like a 19t 540.
get a speed gems 15t. they are pretty good motors for the money. it will be faster than the 12t titan for sure.
also, larger pinion equals more speed. until you over gear the motor and fry it(been there, done that!)
i wouldnt gear up the titan more than you have. it will most likely fry itself.
if you got a decent 15t 540 motor, and a 2s 3000mah lipo then you could do 45mph for sure.
a decent motor will run $30, and you can get 2 lipos and a lipo charger(imax b6) off ebay for $100.