Here is what James at XD-HS2000.com. He sandblasted, bead blasted then painted it "Stainless". It turned out fantastic. It looks very similar to the finish on my 3rd Gens.
Here is a comparrison shot of my M&P Pro 5" with a factory stainless slide. The factory slide is more satin nickel looking, where the paint in "Stainless" is more satin silver.
James also installed the TFO sights, which I have on my other M&P's.
-Bryan