Parsons,
It is necessary that your .375 be reliable with 300 grain bullets for several reasons, not the least of which is that:
You could end up having to sort through some partial boxes of factory ammo that your PH has in camp, because 1 piece, of your luggage was delayed by the Airline Thugs, for 3 or 4 days, with your you-know-what in it.
Not very likely but it can happen.
(Those questionable people lost my wife's luggage for several days on a dive trip to another country but luckily we were able to borrow another regulator and a mask that actually fit her angelic face until hers finally arrived).
However, regarding 270 grain bullets in your .375, that is THEE Classic PG Load, which H&H developed for "The World's Greatest Hunting Cartridge" in 1912 and dern few hunters will have realistic complaints about it.
Even the plain old vanilla Hornady 270 gr RNSP (or spire point if hunting in open terrain) at about 2600 fps, will be effective from bull eland to bull duiker.
(That bullet might not be the best for body shots on giraffe but it will likely be better than a lightening strike for the high neck shot on them).
I am fond of (almost married to) the Hndy 300 gr RNSP at 2400 fps but, I totally get it that most hunters want a bit flatter shooting load....as they say - "just in case".
Most African bushveld conditions never present long shots but, I totally understand what they are concerned with.
The old fashioned bullets I suggest have velocity limitations though.
If you try to push them beyond approximately H&H speeds (Weatherby / Ackley Improved, Spazzeroni, etc.) they can shatter on heavy bone and fail to penetrate.
Keep them below the red line loads in your reloading manuals and they will be a combination sledge hammer and concrete drill for sure.
"Cup & Core" seems to be a negative term with some folks these days but, the .375, .416 and others have built their awesome reputations on them.
Reputations that all these newer, here today/gone tomorrow calibers can only dream of.
I have thought for a long time that if a man could afford but one centerfire rifle, with which to tramp the earth that, the following would be about as close to perfecto as it gets (unless of course grumpy elephant wandered in to wake me from my pipe dream).
The 98 Mauser, in .375 H&H loaded with 270 gr Swift A-Frames and a 4x Zeiss (or similar sturdy brand), not loaded too hot, so as to avoid sticking an empty in the chamber and/or ruin a lot of edible meat.
It'd seem like from bayonet range, out to around 350 yds, or perhaps a tic further, it would be about as perfect for African PG, large and small, as one could pray for.
As well as for N. America elk, moose, grizzly, bison, and lest we forget - Aussie 2000 lb buffalo, also "boar and bull hunting" (wild piggywiggy and scrub/wild ox) as one could ask for.
It (270 gr A-Frame) at least in theory, would do well on cape buffalo (however I personally would prefer more gun) but it seems like it would probably prove my concerns unfounded.
Help, I'm typing and I can't stop.
Out,
Velo Dog.