Jeff, I have several of these as well. They have never been a problem that I can see. I always see them grazing on the LR. Never seen them attacking or mounting snails. I posted a few months back about this and couldn't get a positive ID or anyone that would commit to if it was reef safe or not. Also,
Chuck's Addiction says that all whelks are predators and carnivorous. Mine are still in the tank.
Here is some reading on the Whelk.
Whelks by Ronald L. Shimek, Ph.D. - Reefkeeping.com