While your testing the RPM's with and without the diverter you should test how much residue you get with and without. Unless the diverter is long enough to get the end completly out of the prop wash it will still get on the plane, maybe just more spread out and farther back. The prop wash spirals back and catches the exhaust if it isn't outside this swirl.

For the loss of power and to still have residue on the plane I say they are a marketing item as opposed to a useful tool used by experienced modelers.