So a few questions:
1. Do you believe the caliber restrictions for Cape Buffalo should be the same as the larger African game? i.e. 375.
2. Do you think there should be an exception for using 338's, 35's etc. on Cape buffalo given their effectiveness on other buffalo species around the world?
3. Do you think the 375 minimum is actually too low for hippo, rhino or elephant?
I was hunting with 375 H&H twice, plains game. And hope to hunt buff with 375 H&H in foreseeable future. I shot number of plains game species from steenbok to eland with 375 H&H.
And judging by effect on steenbok, I believe there is no overkill in hunting. Steenbok droppes on spot, just like my eland.
So, my modest view to your questions.
1. Yes. What works for ele, should work for buff.
2. No. But there are exceptions, in some places 9.3x62, 9.3x64, and 9.3x74R are allowed for buff. But if you go in direction of exceptions, for imaginary example like: minimum 30-06 for leopard, min 338 for buffalo, min 45-70 for lion, min 375 H&H for elephant, etc there will be a mess of regulations. Not practical.
In the same time, as I say, there is no overkill in hunting. 375 H&H also has a manageable recoil, and is proven on largest animals. Average skilled hunter can learn and train to handle 375 H&H.
So, in my view 375 can be accepted as a legal minimum for all DG.
3. No. I would say, 375 is accepatable as a reasonable minimum, for average skilled client. Of course, if you can handle and have available 416 Rigby, then take 416.
Remember, there is no overkill in hunting. Robert Ruarks philosophy was to "bring enough gun". So, bring enough gun.
Start from 375 H&H, and eventual going up more in power is up to you, however, make sure you are skilled enough to make first shot with good shot placement in the field.
So, your skill is limit in rifle power, and big bores require training and trigger time at the range before actual hunt.
O BTW, while for many animals you generally shoot in vital zone, which is very large target, for hippo and croc, and sometimes ele, you shoot at brain. Accuracy is important.
The most accurate shots in hunting, are made on hippo and croc.
(Tennis ball size of target for croc, at range 50 to 100 meters, hippo a bit larger brain, but just a bit out of water)
You miss the brain, hit them around, they will dive, you will not see them again, and will pay for full hunt.
So, for brain shot on croc and hippo: power 375 - 416 - 700 NE, does not matter. Brain shot will anchor them, but you have to make accurate shot, and medium power of 375 will be easier to handle and shoot accurately. This is game of accuracy, not game of power.