I have to go with the mag spring as I live in occupied territory, aka California. 10 rounds max here and as far as I know I can buy the 15 rd. spring.
You bring up two very good points with the slide release lever spring and the bumping with the supporting hand. The gun is twenty years old and the only thing I've ever replaced was the recoil spring, guide, locking block and plunger. I think it's time to overhaul the old 92 and I do hold it with thumbs high on that side. Thanks.
Guns don't kill people, the bullets do.