So, if anyone is interested, here are the "efficiency" ratings that I found for some various 7mm cartridges. The idea, only a marginal representation, was to find the most efficient case capacity of a 7mm cartridge.
The numbers listed below are the velocity (in ft./sec.) divided by the grains of powder it took to make said velocity. Ratings were given from the fastest load that could be found from reliable sources a.k.a. published reloading data. (Most of the calibers were found on the Nosler reloading web-site so as to give more consistency to the numbers).
All cartridges, unless otherwise noted, were loaded with a 175gr. bullet. Here we go:
7mm BR (w/ 150 gr. bullet) 39 / 87.8
7mm-08 51.3 / 59.2
7x57 55.6 / 62.5
7x57 AI 57 / 58.7
7x64 Brenneke (w/ 160gr. bullet) 62.6 / 45.3
.284 Win. 60.4 / 52.2
.284 Shehane 63.7 / 55.1
.280 Rem. 64.4 / 50.2
.280 Rem. AI 67.6 / 49.1
7mm SAUM 68.1 / 48.3
Lazzerroni Tomahawk 71.2 / 47.9
77.1 / 49.6
7mm Rem. Mag. 81.8 / 43.5
7mm Weatherby Mag. 82.2 / 43.6
7mm STW 88.9 / 42.6
7mm RUM 106.4 / 34.4
Lazzerroni Firebird (w/ 160gr. bullet) 124.4 / 38.3
Wade Super Seven Mag. Case / 47.0
7mm Gradle Mag. Case / 50.4
7mm Ackley Magnum Mag. Case / 49.2
.284 Durham Magnum Mag. Case / 50.3
7mm GT (from Quikload estimates) 63.3 / 54.9
The first number is the powder capacity in grains of water and the second number is the efficiency as stated previously. The "Mag. Case" cartridges are wildcats form P.O. Ackley's reloading manual vol. 1 so I don't know what the exact volume are. However, they are all made with belted H&H brass or something similar, and are left fairly long therefore they will have larger case capacities.
I know that this is not a complete study in that it doesn't account for all the different loads listed, or "accuracy loads" that could possibly shoot a little faster but, I think it gives a good general representation.
What I am seeing out of this is that with the 7mm bore shooting a 175. gr. bullet the ideal powder capacity (translating into, efficiencey and barrel life) is somewhere between 60-64 grains of capacity.
The only major deviant from this is the 7x64. I don't know if this is due to the 160gr. bullet or something else as I did not put a rediculous amount of time into researching loads. But there it is interpret it how you will.