I don't think it even needs that much, it's quite strong outside Abyss and it makes /RDM a very good sub choice with fencer and the MAB bonus. Somewhere between a 25% and 50% buff would probably do the trick while not overpowering.
Apart from brew I don't think it's realistic to expect it to perform very well in Abyss anyway. Even less realistic after the increases for crits. I've fairly casually gained an extra 5% crit. damage boost from Augments too - further pushing Rampage ahead.
Remember the buff for Bora Axe SE? Little bit of that for a WS that actually has potential please.