The problem with avoidance is that whilst it is fairly decent for TDR it's not really great for anything else. Trading avoidance for mastery isn't really going to boost your DPS as what you gain in Revenge procs you lose in enrage uptime. If you want more DPS get the Expertise hard cap, and if you're willing to lose survivability replace GoVR with GoHtL and forget stamina get more mastery.
But get the expertise hardcap. its good for every situation where you get to hit what you're tanking whether its magic, physical, whatever.