I don't think the grip extensions are intended to limit magazine travel -- they're just there to make the grip more user friendly or to make the gap showing a bare magazine more attractive. (If the grip extension is too long it will keep the mag from engaging with the mag release and not lock into place. That happens sometimes if you run different mag base plates, and sandpaper on the offending part often solves the problem.)
I made an earlier attempt to respond to this topic (in this same message #) and reworked that after response re-reading what I wrote and re-examining my M&P Pro. Without X-ray vision, some of these things are hard to see and understand.
The magazine itself, by how it butts against the slide stop on the front and how it is positioned by the mag release in the grip, seems to be limited in its upward travel by at least those two items. (There are probably other parts of the frame design also limiting the magazines upward travel that I can't see and clearly don't understand). I'm sure part of the mag would hit the underside of the slide if it moved up too high and rounds wouldn't feed properly. for the slide simply wouldn't be able to move forward.
Remove the slide and try it with your gun. I suspect that if you insert a mag with an extension mounted it will not position the mag any differently than if you didn't use the extension (unless the extension was too long and kept the mag from engaging the mag release.
Many of us have used longer (higher capacity) mags in our compact guns, (or 33 rounders in our 9mm Glocks) without slide extensions, and never encounter a problem. (I have a souped up Kel-Tec 9mm sub-2000 that runs Glock 17 mags -- and it will shoot beautifully all day long using Glock 33 or aftermarket 32 round mags without a hitch -- no mag extensions required.)
If something IS out of spec -- that out-of-spec condition would have to allow the magazine to move up and additional 1 to 2/16ths of an inch or more before the ejector or extractor are at risk -- and it looks as though the extractor would be at greater risk than the ejector. But, I've never heard of that sort of malfunction or breakage happening with any gun, but it may be a possibility.