Does the 7RM actually have more capacity, or significantly so? Seems I remember reading that the WSM was within 1/4grn of water to the RM. Not a huge hurdle for a bit of efficiency to make up if we're talking less than 1/3 of 1% difference case capacity.

The "no replacement for displacement" applies to engines because we all run the same fuel, so you're looking at roughly the same compression ratio. Rifle cartridges get to play at different pressures. Really simple example: pistol vs. revolver cartridges 45 ACP vs 45colt, eyeballing it, over twice the powder capacity beneath the bullet it, but I get as much out of 10grns in the higher pressure ACP as I do out of 13grns in the Colt. 9mm Para vs. .38spcl - twice the pressure, half the capacity, little more than half the powder charge, same velocity with the same bullet. Sure, I can +P+ a .38spcl, could do the same in a 9x19.

I'm not doubting that it's possible, even call it easy, to load the 7RM to the same pressure as the 7WSM without having pressure issues, but if we're taking SAAMI + 7% loads for the 7RM, why not talk SAAMI + 7% on the 7WSM? The 7RM taps out at 61kpsi, the 7WSM is spec'd at 65kpsi. Bump the WSM up 7% as well, and I'm sure it'll get faster as well.

I had an RM several years ago in a rebuilt Ruger (remember the skeletonized synthetic stocks?). Eventually got a 7WSM, and would like to have another one now (although it's hard to say either does anything that my 284win doesn't do). Faster? I know factory ammo sure was, and my reloads with the same bullet, loaded to, or just beneath max SAAMI pressures were as well.

End of the day, the gap between the two, whichever is REALLY faster (betting on a case by case basis, they swap heads and tails), isn't so significant that it would make me say that one has a ballistic advantage. Reloading WSM's (or any short, fat cartridge) isn't as hard as people make it sound, and no more difficult than reloading proper headspace for belted magnums.

Personally, I'd prefer the smaller powder charge, no belt, and lighter rifle.