I would say if your gonna paint or powdercoat the centers, I'd reseal them anyway. I don't know a lot about sealing wheels, but I'd say if they sell the stuff that CCW uses at Lowes I probably wouldn;t hesitate to use that.
As for the color, I think the silver looks amazing and it goes perfect with a silver car. As you said, the polished lips will create a little contrast, and look perfect. If you do decide to do black, I still think matte finish will look the best. What I've noticed with gloss black wheel centers on multi-piece forged wheels is that if they have wide spokes, it makes the wheels looks smaller. Since the spokes on yours are both wide, as well as slightly convex, I'd stay away from the gloss black. Again, your wheels, and I'm sure any way you decide to finish them they will not only look amazing but completely unique. I have never seen these wheels on a Mustang before, so this is gonna be awesome!