Running my stock Pro Slot yesterday and a friend was checking it out and pointed out to the pinion bushing being worn and sloppy. He said if you're going to pull the motor down you might as well put ball bearings on the pinion end and the endbell and be done with it.
If you look at the way you framed your question, you were really pointing to a "wear" issue, but asking about a "speed" issue. Certainly, a decent quality ball bearing will provide a longer service life than a sintered metal bushing. The bearing is made (generally) out of steel, the bushing is made out of powdered (usually) softer metal and has a lot of holes or spaces so it can absorb and hold oil like a hard "sponge."
- Cheap bearings (and there are a lot of them around) will have more "slop" than the better ones, but all bearings will have at least some because of their nature. The best bearings have very little. That "slop" can allow the armature to move laterally ("vibrate") while rotating, which could cause arcing. In reality, and with the slower/low cost motors, it's probably not an issue and is probably less than the wobble that happens when either an oilite or even a solid bronze bushing has very slightly worn.
- If you could set up a tightly controlled experiment where you took one armature and installed it in a set-up where the bore of the can and endbell were slightly off from each other, you would find that it was easier to insert the armature if there were two bearings, than if there were two oilites. The reason is that ball bearings by nature have this slight "slop" (the inner race is riding on... balls!, and can rotate a bit). Of course, the above experiment would be difficult to set up because the bore of the oilite and the ID of the bearing would have to be precisely the same, which they most often aren't). I would guess that "needle" or small "roller" bearings can have less of this kind of ability to wobble than ball bearings, but engineers would know for sure. There was a brief time when I was a kid that (I think) Cox sold needle bearings for slot car axles, but they fell out of favor.
- When an oilite starts to wear, the amount of slop can quickly become way more than even a cheap bearing has to begin with, and that wear will accelerate. The more wear, the faster it will wear. The more "slop," the faster other motor parts will be affected and the shorter the motor life... plus degrading performance (slower, more heat, and current draw from arcing, poorer gear mesh at the driven end of the motor) before the motor fails.
- The difference in "speed" between a fresh tight oilite (or even a solid bronze bushing) and a ball bearing is probably not that great, but the bearing will survive much better over time, and especially be able to better handle a too-tight gear mesh on the driven end of the motor. All other things being equal, the bearing will provide better service over the long haul, and most likely better speeds. In slower classes of racing that allow you to use either, you'd have to balance the cost of bearings with the cost of the whole motor. It might not make "economic" sense, when the whole motor can be replaced, but I still think the faster guys will use bearings. It may be also that inexpensive bearings (at something like $6 or $7 a pop) are perfectly fine. At the high end where a motor can cost $400 or so, I think people will opt for the best bearings because they're spending so much on the motor anyway, and a bearing not rated to handle those kinds of speeds can fail in a very sudden and bad way.
... so, bearings are "better" for the most part, but not necessary in many cases. As was mentioned above, a bearing only on the driven end of the motor would be a good "half measure." If cost isn't a factor, the bearings on both ends sure isn't ever going to be a "bad thing," and might even be "the best thing."
I hope this makes things as clear as mud!