Well, if you shoot in the crop mode, you do gain the much better SR which will give you the ability to shoot at a lower speed for still subjects and use a lower ISO setting. The ISO should also be better, so you do gain some positive benefits even with APS-C lenses.
This is my main interest......but having the FF advantage for other lenses and other styles of shooting, like portraits etc. is the added bonus. Basically you have a K5 and a FF in the same body. Add the pixel shift and other fine features and it makes it a great all around camera for most any type of shooter.
Regards!