I am not suggesting the effect you site does not exist but it may not be as detrimental as you may think. Here is another 1905 Kynoch test with what would seem to be the 1250fps Explora load which was Hollands magnum loading also.

Maybe not as accurate as the subsonic load but would provide more energy at 200yds.
I also wanted to clarify ; probably less than 100 Westley Richards Super Magnum Exploras made.