So about the only draw back is I woukd have to snip the ejector off my dedicated lower or get another.
Most 9mm lowers have the ejector held in place with a set screw or something similar and can be removed.
But if you noticed my comment about the Endomags, you can use any 5.56 lower with those. I like them better than Colt or MP5 mags anyway; they have the same feed advantages (still double feed like the regular Pmag, they use the Pmag feed lips) but also have the advantage of the same feel and mechanics as a 5.56 rifle. And they're a lot cheaper than other SMG mags anyway, and don't need a mag adapter block. The standard bolt hold open works with these too, which is often an issue with other 9mm AR mag types.
To be clear - I don't sell these or have any financial stake in them; just pointing out that they seem to have all of the answers we look for in 9mm AR systems,
IF you're using a CMMG or other system that has an ejector in the bolt. For blowback systems, they do make a version that has a mag-mounted ejector, but I'm not real excited about that idea - it's plastic, and you can't eject a round after dropping the mag in that blowback version.
I have a Glock mag lower too and a handful of 33rd happy sticks, but not sure there's any good reason to use it after getting set up with these Endomags. Maybe with one of the Glock-compatible drum mags there could be an advantage, but I haven't tried them and not sure if they have durability/reliability issues in ARs; it seems to depend who you ask.
BTW
@WindstormSCR - the barrel extension in this CMMG system is only different from a 5.56 in the feed ramp - it's got single wide feed ramp that accomodates both single feed and double feed. Apparently CMMG's original design included angled lug faces, but they're building them with standard flat faces. Pretty easy to make the barrel itself using a 5.56 extension if a guy has the ability; it'd be essentially the same as a 9mm DI barrel but without a gas port.
Adding a couple pics of a MEAN Endomag. You can buy them complete, or just the guts and load them into your own stripped Pmags. IIRC they were around $30 each for the guts, and Pmags can be had for $10-$12; a lot cheaper than Colt mags by a long shot. Also, they handle a bit longer OAL than normal 9mm mags; not a huge difference but maybe another .100" or so, enough to open up some more options with heavy bullet subsonic loads.