That distributor comes with the cast iron gear. Regardless of material, all the gears will be the same. Can you take some measurements on the MSD gear and the original gear and see if there are any differences, like thickness of the teeth or diameter of the gear? How about diameter of the bottom shaft that goes into the block? Also, make sure you check the dimension shown in the MSD instructions for the distance from the gear to the distributor body.
If you had to tap the distributor to get it in place, does it also not want to come out? When you take it back out, look on the gear and the bottom shaft and see if you can see evidence of binding or tightness.
Also when you pull the distributor out, take a look down the hole at the oil pump driveshaft. Does it look OK down there? Is there a burr on the driveshaft, like the distributor wasn't lined up quite right with the hex on the driveshaft and galled it up or something like that? After the distributor is out I'd put a 5/16" 6 point quarter inch socket on an extension, put it down the hole and spin the driveshaft to make sure it spins freely. Tape the socket to the extension so it doesn't come apart when you put it in the hole. And don't force it onto the oil pump driveshaft or you might have a lot of trouble getting it back off.
If you can observe some wear once you have the distributor out, or find some differences in the measurements it should clue you in to where the problem is. I have to say I find it hard to believe that the distributor being installed puts such a drag on the motor that it turns over slower. If that is really true, something is way, way too tight in there...