It's a 5% damage taken increase actually.
Focused Will for Disc is 10% per stack in PvP atm. 2 stacks so 20% (actually 19% but w/e)
In 8.1 they're making Focused Will reduce damage by 15% again, but it can't stack for Disc anymore. So you're losing roughly 5% damage reduction. It's a big deal against Turbo but not the end of the world.