I've been trying out Armor Master for a PVE tank build. For him, he rests around 25k resistances only using 3 Reinforced (1 heavy piece for Undaunted) and 4 Sturdy armor traits. Resistances aside, you still need Minor Protection and a hefty sum of CP for the build. I was also using Elude for its long uptime, and proccing resistances well over 35k with Major Resolve.

Outside reaching comparable resistances to my Heavy Armor build, what is noticeable is the missing 8% healing from Heavy Armor. It's something that's not easily replaced with CP allocation. as my tank has full points in both non-slotted passives to boost healing done on self-heals.

For PVP content, reaching resistance cap is easily done with Armor Master. Although there's a stamina drain for recasting Elude for the extra 5k resistances but it's partially mitigated when you maintain Major Protection and toss in a dodge roll for the extra AOE damage reduction to get out of those hot spots.

Despite all this. The OP does reintroduce how effective Armor Master synergies well with the Medium Armor passives. You can be very tanky with all the damage perks. As he mentioned, you only need to backbar it when you need to push resistance cap and s/b is a good setup for that. Plus, you can toss in Pierce and get LA penetration for 15 secs. when you turn offensive.

Nice build.