It's true, Hobie's warranty does not cover "damage by alterations or modifications", but it does cover "defects in material and workmanship". Fortunately one does not necessarily affect the other.
It's a good point though -- a lot of users wonder if making a modification (installing a ram mount for instance) voids the warranty. It doesn't, as long as the ram mount doesn't cause a failure. Many Hobie accessories require modification to the hull, such as Sidekicks. Often, modifications are installed by the Hobie dealers -- doesn't matter.
As for the grips, I've been using them for a couple of years now and find them to be very useful and problem free.