It's not the velo that makes Brian's FB above average, it's the control. He's always sat 89-91 and can touch 94 when he reaches back (I've scouted dozens of his games). Where he's using it effectively is as a set up for the change and curve, and it gives him a pitch for the other side of the plate since the change and curve both break to the same side. Guys KNOW what his best pitches are and know he's going to throw them, so when he catches them trying to sit on the curve/change combo, he'll throw in the FB on the inside right at the knees for a lot of his strikeouts.
Britton throws harder than Brian does, but that heavy sink is what does the work. You'll notice with his spring stats that he's getting a ton of outs with little K's, he's using his defense and that great groundball rate to do the work for him, exactly like he should be. They will be a great 1 and 3 punch for years to come (broken up by having a righty in between them in the rotation).