I'm really wanting to get an FM-15 in the 12.5 configuration. I've been shooting suppressed a ton, and hope to continue to do so. I'm pretty sure I want to have an adjustable gas block, or I'll even go with the recoil spring for suppressors if needed. Anyway thought I'd ask.

Not a Primary Arms related answer but:

As far as I know the 12.5” 5.56 FM-15 upper with 3-position gas block is a Rainier Arms exclusive version. Aim Surplus has the 11.25” with adjustable gas. I have an 11.25” one and while 3 settings are kind of course, it has worked so far adjusting with and without a .30 cal Vox-S.  FM also sells a suppressor-only spring, presumably heavier.