I'm not talking about the full size mag in compact/subcompact guns, but extended off the base of the mag.

I carry a full size M&P and a spare, but thought a few more rounds in the spare isn't a bad idea. Something like this. Figure the factory plate digs in a bit and this will help cushion it some, but I know it can stick out more. And reliability is always a factor as well.

This is for the spares only, not the mag in the gun, for better concealment.

I don't see a problem with that. I'd run some rounds through it to make sure it doesn't cause feeding issues. Why not just add an additional magazine for the extra rounds and be sure that the aftermarket add on doesn't cause problems? I only carry 1 spare, but know people who carry two spares as well.