The message I was getting was the same everyone else was getting who wanted to order from Apple.com. "The Apple store is updating, and will be right back." That doesn't mean the servers were loaded, it meant they weren't done updating the store.
Again, people had more success if they used the Apple store App. Myself and many others COULD NOT, as it didn't have the T-Mobile iphone listed.
The store came up for me, in a browser, on one device (at 12:15 AM), but remained "We'll Be Right Back" for at least an hour on the rest of them. There was some caching issue with the CDN, which is why people were able to get through via incognito mode. The store actually was up around 12 AM though.
Same boat. 7-10 days on iPhone 6 Plus 64GB T-Mobile. It's one thing to have low stock of that model, and quite the other for the site to not work for 2.5 hours..
I think the reason stock was so low was because SIMs are put into the phone as they leave China. Because all of the phones are in China now, there isn't such a thing as a T-Mobile phone yet, just a 6+. Since the app went live, almost all of the GSM 6+'s went to other carriers.
The most annoying thing is WTF they wouldn't let anyone order a T-Mobile phone via the store app. That part made no sense.