I just logged in to check and discovered something weird.
I was at 308M standing in town. (that already includes the base 28% RCR). I took off the belt, and i went down to 159M. I trigger AC with the belt on, i go to 1.3B. I trigger AC with the belt off and i go to 677M.
So that all makes sense and supports your statement that A2 set bonus doesn’t buff toughness.
Now here’s where it gets tricky…
Equip the belt, activate AC… while AC is running, take off the belt, before the AC fades, put the belt back on. I’m now sitting at 2.6B toughness, AC fades i’m at 616M toughness. From this point take the belt off, put it back on, now i’m back at 308M toughness like before.
Now… here’s where it gets very wonky… that snapshot occurred in town. I entered a GR, was sitting at 308M/1.3B – after porting to town and activating AC, the toughness shoots up to 616M/2.6B and stays like that. You can run around the whole rift, (without dying) with the higher numbers.
I could not replicate this same behavior with the Laws of Valor:Unstoppable Force.
Oh and one other thing, take off all Akkhan gear and you can’t do the snapshot.
IMHO, they should just make these interactions compatible. RCR is RCR, whether it comes from gear or a legendary effect. If it affects the RCR sheet line item, it should count towards Captain Crimson 3-piece bonus.