O'Connor praised many different cartridges, including the 7mm mauser, the .270, the .30-06, the .30-30, and the .358 Winchester. From everything I've read he reserved his most unconditional praise for the .375 H&H, NOT the .270, which he considered merely part of a 'class' of cartridges that were very useful (to include the .280 Rem and .284 Win).
He talked a great deal about the value of a relatively heavy bullet moving at moderate speed. He preferred a big round-nose bullet for shooting deer at short to medium range, especially in the woods, though he also kept the legend alive that these bullets 'buck brush' better, which yeah isn't true.
O'Connor was absolutely not a 'velocity nut'; he spends many pages making fun of the Weatherby boys, and the nuttier wildcat nuts, and the rest of the cult of speed. He spends equal time making fun of the grognards who think the 'killing power' of a rifle is equal to the size of its bore--which was true in the black powder days, but no longer true with smokeless powder and high-intensity cartridges.
The 'argument' between himself and Elmer was one sided. O'Connor hunted and shot and wrote about many different cartridges--including 'slow' ones--and Keith #####ed about everything he wrote because he didn't follow the big bullet orthodoxy. Elmer was an entertaining writer and a true 'character', but nobody ever accused him of being reasonable, logical, or very bright.