Just received mine.  Looks nice.  It is an MCX upper with an M400 lower along with the Sig AR lower adapter.  All the adapter really does is allow you to mate the MCX folding stock to the receiver.  It does not appear that it does anything else, other than cover the "Caution" sign on the end of the upper receiver.  Internals are obviously post recall.  Cost me $1125 even on gunbroker, shipping and all.  Only real difference between MCX lower receiver and the M400 is that the MCX lower receiver has an ambi bolt release, while the M400 doesn't.  Naked, the gun weighs 6 lbs. 10.5 oz.  That's a full pound lighter than my XCR.  Light as a feather.  Comes with skeleton MCX stock(which I will be replacing).  Pics included:

Dibs on stock. Hit me up when ready to sell. My MPX stamp should be back soon and I want to use the skeleton vs collapsible stock that it comes with.

Done.  Should be receiving my other stock this week.

So...

With this adapter, would it be possible to use any AR15 lower reciever?  To the best of my knowledge, the M400 lower is essentially a "milspec" pattern lower with some different aesthetics.

This could be a nice way to save a big ass chunk of change on an MCX upper...

Yes, but the MCX upper even works without the adapter.  It just has a glaring sign that says, "Caution" in your face and you can't fold the stock.  This is the proper way to do it.  Sig does sell these separate, but last I checked they were out of stock.  And yes, the M400 lower is just your run of the mill milspec lower.  It's from Sig's basic AR15 rifle.

Yep I bought my hyrbid .300 blk purely to get the 9" upper to SBR with my MCX lower. Now I have another milspec lower that I can do whatever with.

Hey, when I called SIG CS, the Guy told me the Carbine has a Pin & Weld Flash Hider when I asked how the attached the the FH on the Carbine & the Pistol.
I think he is getting the MCX confused with the MPX. I have a MCX Hybrid pistol & Carbine enroute & I'm going to want to run a Can, so it's kinda a big deal. Can you verify it's not a P&W on your Carbine.