Ok, so an update. I took the time after the chain waxing to do an even more thorough job of breaking the links, really working them back and forth. Before waxing, I checked the chain for stiff links. Didn't see any.

First ride, chain on the smallest cog seems better. Not 100%, but I think it required more torque to get it to slip. It still slips though. Regarding the derailleur spring, the XT8000 has a clutch. When it's locked, there's no movement of the upper part of the derailleur...that part is sprung. Is that the spring you were referring to? The hanging part of the derailleur, the ones with the jockey wheel, it's movement is locked, but the tension can be increased on the chain using the adjustment screw on the derailleur body. I adjusted it to increase the tension. Too much, and the shifting lever becomes too hard to operate on the larger cogs. In any case, the increased tension didn't seem to make a difference.

I have another idea. When I replaced the bottom bracket, I used only two spacers, one on each side. The Shimano brochure seems to indicate the use of two on the right side, but I only used one, since that's the way the bike was put together when I bought it. I'm wondering whether the chain is coming in on too much of an angle onto that smallest cog, and a little friction on the inside surface of the cog, may prevent the chain from seating properly sometimes. I'm using wax, so there's little lube on the side of the cog, Had I been using oil, the side of the cog would probably get some lube, (and dirt). Unfortunately I threw away that second spacer.