16" MBP has been fast. I ordered my high-end CTO on Wednesday the 27th of November, and it was in my hands Monday the 2nd of December. Stock models were even faster for many people. Will Apple be that fast with the Mac Pros?
I suspect the average Apple store will have exactly one display model, and may or may not stock one in the back... Pretty much every Apple store got a display iMac Pro, which wasn't hugely less expensive than a base Mac Pro. They also tended to have trashcan Mac Pros, back when those were new (they got rid of them when they were embarrassingly old and nobody was buying them). The display model will probably be running its own configurator, and, if you walk into an Apple store to buy one, you can use the store demo to order what you want. Of course you can order whatever you want (and can afford) online as well, and that will be the common way - but Apple will have it available for in-store ordering (that's what they did with the iMac Pro - the Apple Store demos had the configuration page as their wake-up screen).
They generally did stock base iMac Pros in Apple Stores, but I suspect the base model was a higher percentage of those sales than it will be for the Mac Pro. I wonder if some large Apple Stores will be able to do some level of in-store customization? It wouldn't be too hard to do - a Genius could pop anything, even including a CPU, into the machine. One approach would be to stock a few barebones Mac Pros with no options at all, and then have a (small) supply of popular parts on hand at big stores. Is it worth it to Apple? Since they can build any configuration from a barebones model in an hour in Texas, they could also centralize that operation there and say "you can pick it up here tomorrow - or we'll send it to your doorstep".