Well, here's my 2 cents worth, having been to RSA for only one, but highly successful, safari. I like to seek the "bottom line" when trying to make a tough decision. Loads of well meaning but conflicting advice is to be found above, so what's the bottom line? I think it has several parts:
1) listen to your PH. He knows the area to be hunted and what's there that bites. He said your 30-06 is fine. Talk to him some more. Was he just making you feel comfortable with your likely rifle, and carries a .416 or the like for those cranky cow elephants, or are there NO cow elephants or the like where you will be hunting? That removes the "protect yourself" idea re what gun to take. In the large game farms of RSA where so much PG "safari" hunting takes place, you aren't very likely to find yourself playing Peter Capstick. Instead you will have the time of your life hunting what you contracted for - PG. Not lion, leopard, rhino, buff, etc.
2) what was said above in one post is ever so true: "To some extent romance can get in the way of practicality sometimes." We all fantasize going to Africa with bearers for our rifles and gear, walking for many miles thru uncharted territory, etc.. But your first hunt there, the typical PG hunt, is very likely not to be that romantic. Fantastic hunt yes, highly romantic no. Who is going to carry your second rifle? Not your PH or the tracker I suspect. They have their regular job to do. But again, ask. If your second rifle just sits in the truck or back at camp, as is likely, was the romance of bringing it worth it? RSA gun regs and police inspection are tougher and longer now, and you can bring ammo ONLY for the guns you put thru inspection. AND you must have them inspected by U.S. Customs before you leave the U.S., and have the paperwork to prove it for the RSA police when you arrive at Jo-burg.
3) the need for larger caliber rifle has become less and less nowadays due to the truely superb bullets now available. Even the adage to use the "heaviest bullets" available is no longer true for a caliber like 30-06.
4) Use interlibrary loan to get and read Robertson's "The Perfect Shot" regarding shot placement for African game. The key factor is that the game you are after tend to have much shorter boiler rooms than the deer and elk shot in N. Am.. We are used to putting a bullet behind the shoulder for a kill. It's more like IN the shoulder for African game. A good 30-06 bullet through both lungs WILL do it for PG, just as your PH said.
5) Use a scoped rifle. I hunted with a traditional muzzleloader and found that the PG was extremely difficult to see in the brush country without a scope. The Plains Game were NOT on any plains! My romantic idea on that was quite mistaken, at least in the areas where I hunted.

Aloha, Ka'imiloa