Thats been true, was when I was trying to drive .358's fast. With CNC machining, and smaller companies out there like Hammer will make you what you want.
In a fast 358 mag. Nosler partitions, Swift a-frames, and Woodleigh bullets are good choses. Hawk bullets makes their bullets with different jacket thickness so you can pick a bullet for the cartridge your shooting. They also will make custom bullets to your specs.
I am illiterate on the RUM family of cartridges, is the 375 based off of the 300 RUM originally?
I have a coworker who has a 300 RUM and has taken sheep, mule deer and Elk with it and always thought that a 338 or 358 based off of that case would be a great all around killer, even of shoulders.