Data for the 375 Ruger is not easy to find. I asked Alliant for some info on their Reloader powders and they sent me a Nosler load of 72 to 76 grains RL 15. The max velocity was 2883 fps for the 76 gr load.
They gave no info on barrel length or pressure.
I'm curious if you can get more, but I would not do it myself. I haven't chrono'd my RL 15 loads yet, but I'd be more than happy with the velocities you're getting w/ RL 17, correspondingly reduced for my 20" barrel.
SUspect you're at or beyond the 62psi limit.
BTW...primer shape means nada, unless it's just not there anymore!
Link below is from Nosler's site...good luck!
http://www.nosler.com/index.php?p=15&b=375cal&s=345
