As far as I know, the GI Expert series never used the Power Extractor, so they would not need the new EGW "large bore" replacement. But the GI Expert with a convention extractor is a reversion to before Para went to the Power Extractor, which IIRC was around 2003, well before the move from Canada. I doubt they have the capability to make conventional extractors in North Carolina, so they have to be getting them somewhere. Why not EGW?
I just don't understand why you want to spend money for a "good" extractor with nothing to indicate that yours is bad.
1911s are finicky about magazines. ALL 1911s, not just Paras, and not just GI Experts. If your pistol runs reliably with Check-Mate magazines, there is nothing wrong with your pistol. Toss the Mec-Gars in a drawer, or sell them to someone whose pistol they work in. Stop hunting for problems where they don't exist.
Yes, the tolerances are for setting up the machine to maintain tolerances on the finished parts. Para machines twelve (12) frames at a time, on a "tree" in the CNC machining center. Each frame on the tree is indexed exactly the same, so if one frame is off they'll have twelve that are off. If they were borking frames by the dozen, they'd be out of business.