Completely legal with a 16 inch barrel. My brother in law has one for a 1911. For what it is, it's kind of fun to shoot and fairly accurate. The Glock may be the better choice for a carbine because of mag capacity.
I've seen them listed in Gunbroker, but never saw the kit installed on anything. Hard to imagine what it would look like and shoot like. What was the price on the kit? $200 or so?
I wonder if it wouldn't just be as good to get one of those Hi Point carbines?
But your right about the magazines. You could get a Glock 33 round magazine and have quite a few rounds to shoot out of it.
In the political times to come these will be a very valuable tool. It is not a firearm and therefor not going to be subject to bans. My suggestion is to get one, mine has been working flawlessly for over 5 years. Another advantage, for training, is that many public indoor ranges that don't allow rifles will allow you to shoot these.