So that would actually reduce your head speed. Unless you went to a larger pinion. Though honestly 300 rpm should really matter that much.

Edit:

Almost forgot to add. If you already have all the parts to do the upgrade and the only problem is getting the pinion on, you might want to make sure that the motor you bought has the proper shaft size and do the same with hole for the pinion.

If it was a 2mm pinion it won't fit on a 2.3mm shaft. If it's the right sized pinion and shaft it should be tight, but it shouldn't be impossible to get it to go on the motor.