I used HS11's a few years ago and once set up properly they do the job ok to be fair.
Make sure they the brake pads hit the rim square on and not from an angle, and have the brake pads fairly close in and the pad adjustment on the lever (which if I remeber is an allen key bolt under the lever near the master cylinder?) is at a low setting, (hope that makes sence).
Also , if your running a HS11, I'm guessing they are on a 2 bolt frame/fork? If so (and your not running a booster) then the frame will flex a good un as a 2 bolt frame will not have a built in booster, frames / forks with a specific 4 bolt attachement for Magura's normally allways have some type of reinforcing to stop the power of the piston pushing the frame / forks out when they hit the rim.
Hope this helps.