The 8 pluses dates are a week after the regular 8. T-mobile told me:
Alright thank you for letting me do some research for you. I can see that the device has not shipped as of yet, but the great news is that you have 2 business day shipping
As far as estimated ship dates, it looks like you got your place in line right away which is great. If you check out
http://explore.t-mobile.com/shipping-dates it will give you the dates for all of the ship dates for the preorder. I understand your concern with other carriers shipping the device already, we want to make sure you are getting your awesome new phone as fast as possible! The shipping dates are subject to change, it is possible that the device could ship earlier but not guaranteed. These ship dates are based off of when Apple has advised they will supply us with inventory.