I've been planning to put together something like an SDM-R.

I like the idea of the MGI for the flexibility to change configurations.

Will the MGI offer the same level of accuracy with a good heavy stainless match barrel as a quality forged reciever like a CMT or LMT?

How about compared to a Larue or Vltor Billeted Upper?

Are there any "ideal" fitting dimensions for the barrel extension on a barrel to be used with the MGI upper?

Who would be a good source for an SDM-R configuration barrel set up to be used in an MGI upper (no barrel nut, etc.)?

Well, twl has stated that Advanced Barrel Systems uses an MGI QCB upper to accuracy test their barrels, and sub-MOA groups seem to be the norm for them, so I suppose there's little to no practical difference in accuracy.

It should have the potential for more accuracy-it's free-floated.  I'm using one of the older MGI models as a basis for a RECCE.  Make sure the barrels you buy for it come without the barrel nut and delta ring-you want them totally stripped behind the gas block.  

Quoted: It should have the potential for more accuracy-it's free-floated.  I'm using one of the older MGI models as a basis for a RECCE.  Make sure the barrels you buy for it come without the barrel nut and delta ring-you want them totally stripped behind the gas block.

Don't most barrels come standard with a barrel nut, and why would it be a problem? Can't you just slip it off.

You can, but for me it was a pain to remove the gas block.