I'm using a .308 at the moment. Any advice what calliper I should get as my second hunting rifle. I enjoy hunting i.e. Impala,kudu.....if I may add I'm comfortable shooting up to 400m.
Get them all, how does one person only have one rifle when they have two eyes and two hands. Ok I am not really kidding but money and licensing might be an issue. If that is the case I would go much bigger, like the .375 H&H and then you have any Africa animal covered. The 300 win mag, 300 SM, 338, and 9.3 are all great calibers but the .308 can do everything they can do. To what extent is debatable. But the .308 can not do the larger/dangerous game animals but neither can the other 300 cartridges. Ideally plan on getting more than one additional rifle. As I tell my wife all the time, a new gun is cheaper than a girlfriend so be happy!!!!!!!!!!!!!!
338win mag is an excellent one. The 300win mag will also work well but really is not much more than your 308win for all practical purposes at practical hunting ranges. If you are good with the 308win at 400 meters I see little "need" for a 300mag. The 338 will shoot a larger diameter and heavier bullet for larger game. The 338win mag is also excellent on the smaller game also.
If you are going to want a rifle for DG then a 375 or larger depending on how you handle recoil is in order.
Ft lbs of energy is a poor choice to compare cartridges. Especially when the lesser does the job fine AT THE RANGES THE PERSON SHOOTS. FPS of a cartridge will matter on long range shooting as well as the BC of the bullet. If all you are concerned about is ft lbs then get a larger bore cartridge. If comparing same caliber then by your standard he should get an Ultra-mag or Weatherby.
Then there is the matter of BULLETS. The bullets that one can use now are so much better than years ago that you do not need heavier or larger caliber bullets to get the job done cleanly. I am not talking about using a lesser caliber but that modern bullets of the same weight(or less) will now out perform bullets of the past. That is what Nosler Partition started years ago and it has grown into more top quality bullets.
Then there is the recoil/noise for the small gain between the 308 and 300mag AT THE RANGES HE SHOOTS and most game is usually taken---especially in Africa. Some cannot handle the recoil as well as others and a poorly placed shot with more ft lbs is really worse than a well placed shot with less.
I own a number of mags including a 300win mag but it tends to stay home and I will take my 308win or 30-06, if I choose 30cal, when I go hunting. Surprise: I have yet to not hit and kill my animal with the lesser rifles even at ranges out to 600 yards which is farther than I usually shoot.
Any energy that is expended AFTER leaving the animal is wasted energy. Ideally the bullet should expend all its energy in the animal and just exit and fall to the ground. Sometimes a bullet going too fast will not expand as well especially on small animals or not hitting bone. Just zips thru and does less damage than a slower one with less ft lbs. Kind of like what do you need a 1000hp/200mph car to sit in city traffic all the time and never go over 35mph?
Extra ft lbs can be good on some shots but the 308win has enough to do the job with a lot less noise/recoil and possible poor shot placement.
Sorry I do not buy your argument and I have been at this many years and own a number of hunting rifles/pistols.