DrewMartin
Old fashioned new guy
Yeah, the casting alloy probably has a lot to do with it. The way it machined, throwing tiny, tight curls, makes me think high silicon. Nowhere near the finish cuts normally achieved with solid billet. Instead of cutting like hard butter, felt like I was cutting sandstone. Close examination also shows extremely tiny voids. Final grit was #320, and the black emory seemed to remove all surface scratches. I'll try going to #600 on the next one. Open to suggestions...
If black emory took the scratches out from 320 you're good with the process. I'd still give 600 a shot to see if that brightens it up any but I wouldn't hold my breath. The tiny voids are what's causing you grief. You are cleaning out your polish wheel fairly regularly correct? And not using it for more than one compound? Honestly I think they're pretty great looking and I wouldn't worry too much about the final polish. But, if you're dead set on getting these to shine their best, use baking flour as your final compound.
Last option, make some from billet, you aren't too far from it.