I knew I'd have to do some work to the extractor. As most of the parts in the Sarco kit, the extractor appeared cast or injection molded, judging from the seams. The extractor claw was pretty shallow, but it had the necessary bevels -- well, the beginnings of them anyway. Using a good extractor for reference, I took my square needle file and radiused the bevels, and deepened the extractor notch. I also radiused the head of the extractor so it is a continuous curve instead of a bunch of angles. As a final step, I used a fine Cratex wheel in my Dremel to put a good polish on the surfaces.
The barrel was throated, but the surfaces were pretty rough. I used a Cratex cylinder to smooth out the toolmarks on the feeding surfaces of the throat, and I also polished the end of the barrel hood and the surfaces on the top of the chamber. The feed ramp had already been polished, so there wasn't much I could do there.
These modifications helped. I was able to hand cycle a mag full of dummy rounds into the chamber, but the extractor would still drop the last one on top of the mag follower on the ejection cycle.
I suspected the extractor tension wasn't correct, so I opened the Kuhnhausen book. In the 1911, the extractor holds the cartridge against the opposing wall of the breechface, not back against the breechface itself, like in many other designs. So the notch in the extractor is the surface that provides tension, not the claw. I put a slight bend in the extractor and reinstalled it. This seems to have fixed the problem, I can cycle a full mag through the gun by hand now.
I also realized I needed to do some more fitting on the ejector, as it still seems to be binding on the slide channel. It's not really noticeable until everything is installed, but it is definitely binding.