I just did a minor grip reduction on my Gen 3 G26. This consisted of removing the finger grooves and filing down the checking where the fingers rest and the checking on the lower part of the backstrap. I made sure to remove only the raised areas and to not take off any more material than necessary. The the grip walls themselves were not made significantly thinner.
Afterwards, I notice that the frame flexes laterally without a lot of pressure applied. This is with the slide removed and no mag. There is little to no vertical flex, however. Is this normal? I'm wondering if removing the finger grooves made the frame less rigid and if it's a reason for concern (reduced accuracy or a possible failure of some kind). Of course, with the slide on and a mag inserted, the flex is not noticeable.
To be honest, I never checked the amount of frame flex before doing this reduction, so it's possible that it isn't any greater than it's always been. If necessary, I'll fill in the space behind the mag well with epoxy to restore some structural rigidity (so far I haven't done this, since I didn't remove enough material to go through the backstrap).