Hello Steve,
From the ballistic viewpoint, I do not agree about the 9,3x64 is a quantum leap over the 9,3x62. The difference between the 9,3x62 vs. 9,3x64, HANDLOADED to the same pressure in rifles with the same barrel length is not more than 100 f/s with almost all bullet weights. There are not magics in ballistics because of a "name"! Another thing is to compare factory ammunition because this two cartridges are loaded at different pressure standards. Something very similar happens comparing FACTORY cartridges in 7x57 and 7x64 vs. handloads for both, at same pressure and barrel length.
Also, all else being the same: barrel length, pressure, et al, the 9,3x64 is NOT a .375 H&H. It falls just in the middle between the 9,3x62 and the .375 H&H.
I said this because the case capacity of the three, 9,3x62, 9,3x64 and .375 H&H are, roughly: 74 grs, 84 grs, and 95 grs in that order.
So, loaded at the same pressure in barrels of the same length, it impossible to get the same maximum safe velocity of the .375 H&H with the 9,3x64 Brenneke with same bullet weight. Of course the same velocity can be approached using 286 vs 300 grs respectively.
Anyway, I like the 9,3x64 the best of this trio including 9,3x62 and .375 H&H! I have both, a 9,3x62 and a .375 H&H. If the source for cartridges and cases for the 9,3x64 were more common, or easy to get, I would happily replace the former two by a 9,3x64!
Good luck!
CF