.. do
they use 15-lb felt, or naked wood?

No, they still put the felt on. Tyvek just goes on as extra insulation
around widows and doors, in addition to the felt. At least in my case.

... the more convinced I am that McBuilders
are to be avoided, in favor of local independents.

We don't really get a choice. Builder bought the land and is selling
the lots with a choice of models in the development. All these upgrades
are just a way for sub-contractors to make money. The builder hires
them for next to nothing to do standard stuff. For example if you do
not buy any tile upgrades, the tile guy is lucky to break even. But a
single upgrade to bathroom tile, listed for $800 (just to put down
anything other then the cheap white tile) will give the guy a decent
profit, since it will only add about $100 to the cost of the job.

We got very few upgrades, since the final price will affect the tax
amount. We will redo the kitchen and floors after closing for the same
amount that builder would charge, but with better materials , more
choice and no effect on our taxes.