I think this idea is sound. Obviously if you're doing things in your head then you wouldn't need a mod to impose these kinds of limits on yourself, but for me, I can be pretty pathetic at even basic counting and I would find this kind of mod helpful if I wanted to force myself to play in a more tactical or realistic way.
For example, let's say I got into a long fight, and I have with plenty of ammo for all of my guns.
- in a vanilla game, I could fight until I ran out of ammo. The only things I care about are if I reload at a bad time or if my guns are gonna break.
- with magazines, I would have had to prepare my magazines beforehand (possibly automatic by waiting one hour, or staying out of combat for a while, etc.)
- with encumbering magazines, perhaps I'd only have 5 magazines that were readily available for my equipped gun, so now I have to think about conserving ammo.
- maybe some of my enemies have big armor on, so hopefully I have 1 or 2 magazines of armor-piercing rounds.
- if I reload often, switching between these same magazines, this essentially lowers my rate of fire and makes me more likely to get caught defenseless. So to reload or not to reload becomes a bigger choice.
- if my magazines are getting empty, then I'd have to consider switching to another gun (assuming I had prepared magazines for that gun too) or maybe a melee weapon
So yes, I can see how this kind of mod would affect my gameplay. Personally I think it would be pretty annoying if I weren't running a realistic or hardcoe kind of playthrough, but if you want to play with magazine-based reloading, why not?