I wouldn't stray away from a breaking the shoulder shot with 9.3 as long as I will be confident in hiting.

IMo it all boils down to shot placement and adequate straight penetration - one should invest in 1. shooting practice, 2. coming closer, 3. use better aiming device. The rest is all about straight penetration - therefore the right bullet should be used - and here is where you can get some ease with bigger calibers - 9.3&co. calibers palette is narrow but it gets broader when you go up with caliber. The rest is as Adam said - bigger bores make bigger holes - therefore on body shots buffalo usually succumb faster - tho with latest generation bullets, the velocity comes in play...but one should never forget about obstacles on the bullet's path - no doubt bigger/heavier/slower/bluntish is better...Just my 2c - I may stand corrected