I'm using a Ultegra 6750 crank with a KCNC BB30 adapter and have never had any problems from it. Very easy to install, but you would need a BBT-9 (or similar)BB wrench and a torque wrench which a LBS should have, this unit requires 35 ft lbs of torque.

I have never used a FSA crank, so as far as it being stiffer I wouldn't know. The Ultegra crank is very stiff to me..

http://fairwheelbikes.com/kcnc-bb30-...et-p-2926.html