We used to rum 90+ (93gr?) grains of AA8700 in the 7stw with 140's... It ran just fine; we eventually switched to rl powders though. AA8700 (and 869, 870, 872) will work in a pinch- I have a keg or two of the stuff around- but the stuff suffers with the same issues as all the slow burning ball from 20+ years back.
According to my interpolation of the published hodgdon reloading data online, US869 is probably one of the best powders you could pick. ~3100fps. I use US869 with 180's and 168's in a 7RM to great effect and a 7RM eats 10-20 grains less powder than your 7STW. So, your STW is even more overbore than my 7RM and would want an even slower powder or a lighter bullet so it all makes perfect sense. It runs a little dirty but I still like it.