Hmm. Seems like one possible step in the right direction is prevent sewing items into items on the strapped layer, though I should point out I have sewed pockets into a backpack IRL. In that instance was for better organizing stuff for lack of enough compartments, not for more storage. And we don’t really have “better organization” as an in-game effect, only as the rationale for that one trait that multiplies volume.
Sewing layers into clothing should probably reduce environmental protection, I would assume. Now presumably sewing nomex into an item would logically affect fire resistance, but other than that EP would have to go down whether you’re adding pockets or kevlar. Poking holes into an NBC suit is generally a bad idea. XP
As for storage amounts…hmm. Item volume is actually a tad arbitrary now that I think about it. Maybe multiply some fixed bonus by the number of body parts covered, then multiply THAT by coverage percent? Or basically, make coverage percent be a reducing factor.