OP: Lets make this simple for you. If you are mainly concerned about Windows Phones that work on T-Mobile LTE then these are your options:
1) Any T-Mobile branded phone would give you LTE/HSPA+ on its network, period
2) Any ATT LTE device would give you LTE on T-Mobile, regardless of whether the area is refarmed or not, only T-Mobile should have enabled LTE. That's not the case for HSPA+ though. You need to be in a refarmed area to get HSPA on a ATT branded phone. When I say refarmed that means T-Mobile has enabled 1900/2100 MHz for HSPA, if not then it's mostly operating at 1700/2100 MHz which is also known as AWS.
3) The Lumia Icon from Verizon would work the same as any ATT LTE device but it's limited to LTE Band 4 only so if T-Mobile is not operating on Band 4 LTE in an area then you won't be getting LTE, same is the case with for HSPA for a refarmed area
4) Windows Phones from Rogers, Telus and Wind, Canada and phones from Latin America (Mexico, Brazil, Argentina, etc.) support almost all frequency bands for T-Mobile so they would be your second choice outside of the T-Mobile phones and before ATT phones.
5) Any other international Windows Phones would give you a max of HSPA in refarmed areas only. These would be your second last option before Verizon branded but after ATT phones