Now that the 190-195 bullets are readily available in 7mm, I believe there is absolutely no reason to use a twist slower than 8"; unless your only pill will be light for caliber.
I put my new stw together with a 8" twist shilen select match barrel, and she shoots 140 grain ab's as well as she shoots the 195 berger pills. I wouldn't put a 110 grain varmint pill in her (it would likely tear apart), but that's not what this rifle is for.
If you build the rifle to the 9-10# range, you will not notice the difference between the 7stw/ 7x300roy and a 7rem mag. Built lighter, she may be a bit more jumpy than a lesser 7mm.
The rum can be a pussycat too in a slightly heavier rifle; I just picked up a rem 700 long range 300rum(wrong handed for me) and the first couple of groups showed good accuracy even with zero load developement, with recoil in the noticable but no where near punishing range. The rem lr was purchased so I can tear down my left hand bdl in 300 rum when the barrel finishes failing in the near future. I am planning a 338 rum/edge type build for her. A 10# rifle tossing 300 grain slugs above 2800 fps is the plan.