If you're only able to use it as the break down with the 500 siaar plus Royal Compound box finish, I'm guessing you'll want to go down to maybe a matte 1500-2000 to start. I did something similar with an IQ Tour Pearl back in the day and it transformed that piece from a ball that I really didn't care for to one of my favorites of all time. Likewise, I've gone as low as 500 grit on lower-end pearlized pieces like Tropical Heats, and it's made those balls immensely predictable and beneficial to my arsenal. Long story short, if you don't like the look you're getting from your vintage Rhino at this point, there is no harm in changing the surface on it. You may be amazed at how much better a ball fits your game once you get the surface where you want/need it.