Now we all know you like your STW
but barrel life is not going to be all that much better than the RUM, all else being equal such as pressure/heat and rate of fire.
When it comes to efficiency, anytime you climb the case volume ladder in the same cal, you're gonna loose efficiency. A 7-08 is going to be more efficient than a 284 Win which is going to be more efficient than a 7 RM which is going to be more efficient than the STW which is going to be more efficient than the RUM. With the 7 RUM, you do have a very overbore case which restricts the number of good powders that will work well in it but RL33 and RL50 should do very well.
Same goes with the 30 cals. I would agree that the 300 RUM is a little more efficient than the 7 RUM because it is not as overbore, but IMO the 7 RUM is a great cartridge to push the 180 and 195 size bullets. It takes more powder but that's the price you pay.