Unlocked versions of the iPhone 6S/6S+,SE, 7 and 7+ purchased in the US all have Band 12/Extended Range LTE support. Unfortunately the 6/6+ and older phones do not. All depends on where you are as to whether it makes any difference at all. Where T-Mo uses Band 12 it makes quite a difference in in-building coverage but it doesn't exist everywhere.

Unfortunately Apple actually has 3-4 different models of each iPhone that offer support for differing LTE bands depending on the carrier for which they were intended. IIRC, all the US carrier versions and the unlocked version of the aforementioned offer Band 12.

EDIT: Definitive list here