Hi there, there was one season where we had to use M7 screws as we had issues with the inserts stripping or the M6 screws breaking off. Finally we found out it was not the screws, but the inserts which did not have the same consistency in the nylon - anyway, it was mostly easily solved - if you currently have M7x28, then the holes are wide enough for M7, so I would stick to those. M6 will work, but probably not tight enough, especially as you do put a lot of pressure on the straps when riding a Skate or Freewave.