Special Atk is its own modifier, just like Character/Assault is and Elemental is. If you have a proper Grid for Assault modifier, you should notice a significant increase from using SSR Diabolos. If you don't have much of a Grid yet, then the difference between 90% Character and 100% Special isn't much.
SSR Diabolos is really tough to make work though, since you can't give proper Assault/Elemental to your two off-elemental characters... not to mention what happens when somebody on your team dies. It is possible to make work, but unless you know exactly what you're doing, I'd recommend not even trying.I didn't watch that, but wasn't that MagicSpice's YouTube channel? So I wouldn't trust that information for longer than what I can throw it.