A shouldered wear ring protects the suction housing in the event of a bearing failure. When a thrust bearing fails, typically it will force the impeller forward and start chewing up anything in it's way. In the old days before shouldered wear rings, it would eat up the suction housing and it would be ruined. The "shoulder" on the newer wear rings takes that hit now if and when a bearing fails and saves your housing. As GT said, shimming the impeller back is needed in 95% of these installs. Impeller shims normally come in a variety pack of thicknesses so you can achieve proper wear ring shoulder to the leading edge of the impeller clearance.
Shouldered wear rings are perfectly fine in lake boats and even in river racer applications. Full blow drag boat guys don't usually run shouldered wear rings because they like to get that clearance from the housing to the impeller super tight for maximum efficiency. They really are not concerned with wear since they are going very short distances at a time and are constantly having the pump apart making improvements.
