Are the motors you replaced similar to the ones you bought new. The windings in the motor may be the big difference which is causing the speed reduction. you may need more voltage at this point if the new motors have more windings to see a better speed gain. Bigger motors isnt always neccesarilly fast. Other controlling factors are the internal windings of the motors. Larger motors are more torque but with a lot more windings it has less RPM. my Villain EX uses two Monster Max wilds which are 19 turn so i get more RPM then the stock Titan motors which i think are 23 turn.look at the pics on my webspage.

http://blackwidowsinj.tripod.com/MeBluEyz99