I'm surprised not to see anyone also mention Heart of Stone being the weakest on-demand mitigation cd by far, sitting at only 15%. When you look at other options, RI is 20%, TBN is a 25% shield and Sheltron's block is already providing 18% in crap lvl 80 starter gear and it will only grow(deltascape block value was already around 24%, sigma 27% and alpha 29%, it seems to have started at 22% in AF3 gear). That on top of also having a really bad invuln and pretty underwhelming "fluff" cd.

As for the self sustain:
Aurora is literally same value and cd as Equilibrium, just a HoT instead of direct heal and can be used on allies(but that doesn't help you as MT).
Brutal Shell is 150 potency heal, plus same value shield, which is effectively just 300 potency worth of sustain... That's only 50 potency more than Storm's Path and exactly the same as Souleater, while PLD has their passive block chance as an "equivalent" to those. Brutal shell giving a shield instead of just a heal is only flavor - it's good flavor, but don't think for a second it makes up for any holes in mitigation.

GBN seems to have good utility(although pretty sure that PLD still does much better in terms of that), but MT side looks really, really grim unless things are changed for launch.
That wouldn't be a big issue if SE decided to balance WAR/DRK as MTs and PLD/GBN as OTs, but PLD got a significant buff to it's personal mitigation in SHB to add more role flexibility.

As for giving a damage benefit to Superbolide - that's an awful idea. It doesn't solve anything in terms of GBN's survivability and actually hurts it, by encouraging to line it with burst for more damage. Tying dps gain to mitigation has always lead to that mitigation being sacrificed for dps in the past.