I was wondering if anyone had any fics where Spike's, um, bits change when he goes into game face? Preferably if there are D/s themes with Spike as the dom. Thank you

"Cold Day in Hell" by Schehrezade and RSK has B/S discussing that particular change when he vamps, but doesn't have your other requirement. It's actually on the fluffy side, but still an enjoyable read. You can find it at EF.