I like the 225 Sierra for my Whelen's. I've had great luck in accuracy and performance with it.
This year a wounded a Oryx with my 7600 Whelen with a 225 grn Hawk bullet. I cant claim failure, but we're almost certain the bullet went exactly where it should have gone and we only found blood for about 300 yds. After about a half mile we found the group of Oryx bedded and jumped them, looking like nothing was wrong with them.
Again, just an anecdotal. These animals are known to be tough as hell, so it might have just been oryx being oryx. It was the first animal I've lost in a long time and it sure did burn.
-Jon
They are tough only when the bullet does not hit the vitals. Since you did not recover the oryx,it's all speculation.
I put a 270 gr Swift A-Frame from a .375 H&H in the wrong spot on this oryx and had a tough time killing it. It took an hour of following it up before the killing shot was made.
IIRC,it took 6 rounds altogether from my rifle and the PH's to bring it bag.