Single-piston on the rears, but 4-piston on the front Brembo for a stock '11.
Stock are not that bad IMHO. I've stopped on a dime from 125+ MPH on the track with no issues, running an aggressive pads.
Getting more stopping power depends on the pads in most cases. It's a dust\noise\heat compromise...Street brakes are fine in for moderate use (low temps), produce little dust and are very quiet. But they don't work well under extreme temps...go with racing brakes and you get dust, noise, but the stopping power is incredible when they get warmed up. Some manufactures market a 'compromise' street\track pad.
You can upgrade to the '13-'14 6-piston Brembo setup for much less cost than big brake kits, but it will limit you going to a smaller wheel (might not matter unless you are doing road course work). Anything bigger on the rear is a not worth it...Bias of braking is 80/20 to the front. You can upgrade the rotor size in the rear for looks and a little better heat dissipation for cheap.
Unless you are doing completion road course work or like the bling factor, I'd save my money on an aftermarket kit and get a pad that suits your driving style...Just my 2-cents.