Personally, I'm not interested in a mixed manufacture upper/lower rifle set up.
Part of the appeal of this group purchase is the collectabity factor of the final product, which suffers in the case of a mixed rifle set up.
That, and my own personal firearms preferences and collection intentionally does not include mixed-parts rifles.
I suppose the bottom line price would be better under such circumstances(mixed uppers/lowers), and if that's the highest priority here, then so be it.
I am more interested in a unique, complete rifle from a single manufacturer. I see this as a rare opportunity to add a truely special firearm to my collection, and though a good price is important, my desire for a unique collecter rifle is more important.
I don't see any reason why, say Bushmaster, couldn't do a group order of complete rifles, at a very reasonable price. The money "saved" by mixing uppers and lowers creates a rifle much less desirable in terms of long term value, and the initial money saved would be lost in its future worth, negating the intended savings.
I am one opinion and nothing more, but this is MY opinion for what it's worth.
Others may or may not agree, just my .02