Uhhh. Bad idea. Sieg's mode gauge reduction rate buff is very different from Gil's flat reduction, and is infamous for being... very ineffective. It boosts your reduction rate from dealing dmg for like, ... nobody really knows. If it takes you normally 10 turns to stun the boss from rage, it -might- help you reduce it to 9 turns. ...Provided that the buff lasts 10 turns (It lasts for 2).
You are probably better off sticking to Arthur for all time right now. Anyone but Sieg.