Get the gr2 stuff for sure. Take the calipers off, and cover the seals. Run a flap grinder wheel over the caliper to smooth it out, and use several coats of GR2 over it after thoroughly cleaning it with alcohol.
It will look better than most powdercoating. Just PC'ing over a rough caliper will give so-so results...unless they crank it on thick, but that would chip easy.