I would be interested in seeing some of your findings, if you have them documented somewhere.
How did you determine the "poorer grade of material?"
What part of the stud are you talking about in regards to polished? The tapered part, the ball and socket?
Im no materials specialist, but my findings were as follows:
1) Ball stud: The ball stud itself on the OEM part has a polished surface that appears to be made from a higher grade stainless steel that is likely hardened. The movement was smooth and fluid. In comparison, the Raybestos/Sankei/NAPA part seemed to use a darker metal ball that was LESS consistent in appearance and did NOT have a polished surface like the OEM part. It almost looked like a cast iron part. The problem with this was that the movement of the ball in the socket was rough and not fluid. Moving the ball around with my hand caused visible scratches on the ball surface due to the poor mating between the ball and the socket. Strangely, the Upper ball joints from Raybestos did NOT have the flaws that the Lower ball joints had and were actually comparable to OEM quality (polished stud, smooth movement, higher grade steel).
2) Socket: The socket in which the ball sits on the Raybestos/Sankei/NAPA also appeared to be made from a lower grade steel. Once again, it looks like a cast iron part. The material was darker in color, with a less consistent texture. In comparison, the OEM socket has the same qualities of the ball. Stainless steel material that just "feels" more finely machined and hardened properly.
3) Rubber boot: The boot on the OEM joint is a noticeably thicker and tougher rubber material.
4) The Good: The ONLY good note about the aftermarket joints is that they are greasable. I personally am not a fan of sealed components, but I guess it's easy for most people to neglect servicing these parts so a sealed design makes sense for the majority consumer.
Once again, I am no expert on the matter. I would invite people with aftermarket joints to take the boot off and make their own comparisons. The real troubling part is the rough movement from the poor machining/material choice which causes scratching and wear on the surface of the ball. If I can cause those scratches from moving it with my hand just a few times, imagine the beating that ball is taking from 1000's of KMs of driving. As soon as I unboxed my Raybestos joints and felt the movement, I took them back so that I could buy the proper OEM part instead.