|I've had many sets power-coated as this lasts the longest, resists oil & grease and looks the best (IMNSHO)
Most of the sets were stock unpainted aluminum originally. I've had a couple sets that were satin black stock (not totally flat, but with a little sheen).
If you want the stock aluminum look, go with a basic flat silver powdercoat. You will barely see them once the plenum is back on. Satin black powdercoat if they were black from factory.
Speaking of plenums, if you want a clean stock look, pull everything off the plenum and have it bead blasted and clear coated.