As far as I can tell, from personal experience, what 500 grit + Rough Buff will get you is determined a lot by how much RB you apply, how hard you press and for how long. The abrasive breaks down into finer and finer particles. So you could get any where from 3000 to 5000 grit; but once you get above 4000 grit, the surface begins to shine and RB acts like a true polish.

I'd suggest sanding to 4000 grit and adding Snake Oil on top of that. Like all polishes, the more you apply, the higher the gloss level of the polish and the finer the surface gets. Snake Oil just allows you to get there slower in finer steps than more abrasive polishes.

Although Beans recommended, for normal use, to use no finer than 1000 grit under Secret Sauce, you could, if you want even more length and a finer surface. It's one of the few polishes that have enough abrasive to polish urethane balls.