I get it that Hornaday is going for max bang for the promotional buck by creating a common brand identity for the FTX . Some of them make a meaningful improvement over existing loads. But a 325gr .458 bullet is still an inherently stubby bullet. It has a bit more horsepower than the typical RemFedChester 300gr JHP @ 1800-ish.
With a Marlin you can safely propell the 300/325/350 jacketed bullet to vels higher than the FTX (if desired) with several different powders.
As mentioned before , a .45-70 with cast bullets at black powder vels can penetrate a bison at 1000yd . The light bullets at higher vels is just to try to flaten the "point blank distance". But it's not like its possable to make a .45-70 shoot as PBR flat-ish as a .30-30 , much less anything of '06 class.
I wasn't around then , but in his writings Elmer Keith made frequent mention of the changed playing field when 2400 was introduced in circa 1935 .
And while today I would recomend other powders , I had a nice load for 300jhp and 2400, and I'm sure its as good as it used to be.