I am using a 6" bench grinder that gets fairly hot but as long as you dont see smoke than it should be ok. Make sure that you are using quality buffs and compounds. When I switched products, I got incredible results.
With my 6", I am flying through my parts. With an 8", you should be as well. Make sure that the parts have been properly prepped by sanding thoroughly. This makes the polishing process a lot quicker, easier, and smoother and you will get the desired mirror finish that you are looking for.
MM