Sinistral handed shooter here.
As many have said, using the left fore- or middle-finger to release the magazine isn't an issue and many of us look at it as being safer than using a thumb as it practically ensures a finger being removed from the trigger.
Slide releases set for right handed people also work great being released by the forefinger. And of course, all the Glockaphiles will tell you to slingshot the slide anyway and not use the slideSTOP!(tm).
The only requirement I've ever hand is that if there's a thumb safety it be set up for ambidextrous use. In that regards there isn't much better than the FNX series. If I had looked at them before getting my M&P it's very likely I'd have one of them in my holster instead.