Same issue for me for my next upgrade...as I've increased power on my NG900SET considerably. So need the stopping power.
The 4 piston is the best option if your talking an overall improvement, but upgrading to stainless lines would be a whole lot cheaper, in the short run.
Just finished my stainless install, and it really does feel better. Looking at Saab Saviors front wheel 4-piston setup.. A bit costly, but might be the ticket on track days at Summit in W. Va.