I'm not so sure that they're a "slice killer". Slices are not always caused by inactive pronation (rolling) of the hands, they come from swing faults that range from slightly serious to bloody disasterous. No grip will ever cure that.
In any event, assuming you don't have a swing fault that is causing the slice you want the right hand to roll over the left through impact. By holding the thick part in your right hand is actually going to
discourage the pronation, not encourage it.
I wouldn't fit them or encourage them because if you slice, the grip is the least of your problems, not to mention the fact about the right hand holding the fattest part of the grip! 