You have to use the sleeve or you will over-insert. I'm a dumb-ass and didn't even try a 15-round PPQ magazine with my SC. I've now returned my SC to Walther for repairs so can't try it at the moment.
As far as I know and can tell, the SC magazines have a stronger spring in their magazines than regular PPQ magazines. You can tell a 15-round magazine is specifically for the SC because it's stamped "SC" at the bottom. The reason for this is that I've heard that the SC slide is lighter than a regular PPQ (because some internal slide parts are plastic vs metal) and cycles faster. So the magazine needs a stronger spring to feed in new rounds fast enough.
So basically, if a regular PPQ magazine fits into the SC with the sleeve, it may work. But you may experience cycling problems. (Haven't tried it myself, though I should have.)