I guess technically, I couldn't tell you that I KNOW mine grew, but I can say that they moved into "new" shells - when the hermits arrived from reeftopia, they were living in small, darker colored shells. My current tank's substrate has a couple of bags of crushed coral/shell mixed with a bag of aragonite - within a matter of days, I saw hermits that were inside of the white bleached shells that were once just part of the substrate.
When I first got into reefkeeping - a few weeks after my first tank cycled, I noticed one of my BIG fuzzy hermits (I now know these hermits aren't reef-safe) crawling around without its shell and felt as much sympathy for it as one can feel for an invertebrate the size of a marble. I went to Cichlids & Salt and John hooked me up with a handful of empty shells - problem fixed.