Ive only done this once, but...
You proboably dont need to drill it out. Often, this "slit" is a separate part from the gear.
On the servo's I modified, there was a bearing preventing me from popping it out, so I just clipped the pot head off instead.
There may also be a mechanical stop, look for a pin on metal gears or a plastic nub on plastic servos.
Read the tutorial on modifying servos!! It has basically all the info you need.