8x60 would seem to be to much gun just for a 16 to 20 lb animal, if that was all you were going to hunt. Especially for a full body mount. Might do a bit more hide damage than you would like.
I used a 218 Mashburn Bee on my Steenbok and it was plenty gun. I just made sure I had 45 gr. soft points and not varmint bullets.
Heck, I shot my Sharpe's Grysbok with a .416 Rem. Cape damage was minimal as I was using solids. My guess would be you'll see minimal bullet expansion on a Grysbok with most bullets. I'd just stay away from the high velocity rounds.
Here is a Cape Grysbok I took this September in the Northern Luangwa. I shot him with my 375 H&H. There was some hide damage, but very repairable. I'm going to do a life size mount. [image][/image]