Whoa! That's a real boat anchor!:) Seriously, I would try to get it lapped down to a finer grit. This week there is a auction of a granite importer and they have a bunch of wheels, 4000+grit, that fit on what looks like an overpowered angle grinder. I hope to pick up some of each. Maybe I can send them to you to use?:) Or maybe you have a granite countertop fabrication company close to you?
As usual, I am envious.
Hope this helps,:)