The old mutt's dos centavos on velocity:
No major factory .375 H&H ammunition is going to be loaded so slow as to create any problem bagging the animals the bullet was designed to be used on, these days.
Woodleigh Weldcore is not the world's toughest bullet but, it is far from the softest as well.
Even if Kynoch is not loading the 300 grainer to full pressure, (and I do not know if that is or is not so) I would bet it's leaving a 24" barrel at least 2400 fps and probably faster (Remington, Federal, Winchester have always been around 2550 fps with their various 300 gr bullets).
You can sleep well tonight knowing that a 300 gr bullet, in .37 caliber, leaving the muzzle of your rifle at about 2400 fps is extremely effective for heart/lung shots on game animals, large and small.
And with same, you will be able to keep meat in the shed by aiming straight at the vitals on even smallish ones like deer/impala size game, from 75 yards/meters to 175 yards/meters.
At 250 yds, to about 300, you begin to have to aim a tic high but plenty of proper practice at home makes this into child's play before you've traveled half way around the globe and finally close with your quarry.
The Woodleigh 300 gr softs are however not fool-proof, in that they are lead core and although bonded to their jackets, they are nonetheless prone to fail if driven at some ridiculous velocity, such as cartridges like the .375 Ultra-Mag and such, can easily launch them at.
The original H&H, I will guess is not capable of driving them too fast without flattening your primers and unacceptably stretching your brass.
For a soft to use on buffalo, I would suggest the Swift A-Frame but, for any other species that you would normally use a soft on in .375 H&H, the Kynoch I expect will work as well as any and better than some.
Cheers,
Velo Dog.