I took a Shadow RW into the FFVI ++ battle last night and was super unimpressed with that SB. I'm glad I didn't waste mythril trying to get it. I used it on both front line characters with FFVI synergy native 5* gear, with shout and both armor breakdown/full break on the boss and it was only doing around 2k a hit. Meanwhile my 50% HP Locke pulled out nearly 6.5k per hit with Valiant Strike. I knew there was going to be a difference but I didn't expect it to be that big.
Either way, now that I have Shout, SG, Valiant Strike, and Saint's Cross/Saint's Fall, melting these bosses down is ridiculously easy. I don't even bring a healer anymore.