800 is going to give you a foggy looking mirror. I will look like a mirror right up close but further away than a few inches it will look blurry and frosty.
What you can do is use a cork belt and then load it with green buffer compound (half speed on the belt grinder or it all flings off). Use the cork belt loaded with green buff and it will take it to a nice clear, "black" mirror in a few minutes. You will come out of the shop looking like a green Martian but it works very well. SRJohnson uses that technique on his mirror finishes and he is the master of mirror....