I've probably installed a couple of dozen of the Karstens to various composite stocks. The last one a few of days a go on a Springfield M1A. They tend to work quite well for an add on.
The cheek piece is installed to provide clearance so the bolt can be removed for cleaning. They are made of an ABS type material and too me don't feel like they affect balance much. Of course, I don't have much of a sensitive side so I wouldn't notice such things.
