Shinbone,
My RBL16 grip is nearly round, this does give it a full grip and it could be thinner side to side. I've considered the same work as you. It's not that the grip is too large. It's just not the most comfortable shape. Like you I don't have any knowledge of what is proper other than what feels right to me. I think I would make some mock ups from soft wood to try first.
Another concern is how much can be removed from the side before the draw bolt hole is weakened or worse!
I restocked an Ithaca/SKB 20ga that is a very similar gun. I was able to get the grip into a comfortable size and shape for my medium sized hands. That ended up being somewhat egg shaped, the larger end at the bottom. No problems with the throughbolt yet, fifteen years in use.
Chief
