It's been awhile since I was on the KTOG SU-16 forum. I had the impression from your statement that my -16 could be converted to take the 300 BLK. Seems the factory is using the basic frame and putting a 300 BLK barrel in it, along with a compact forend, to make the SU-300BLK. Still won't be able to swap out barrels/forends like the AR.
That's contrary to what I read, but KTOG is more likely to have the right story. I should have thought to look there.
Oh well, another excuse to buy a gun :-).