T-Mobile is lighting up LTE around the country, but they are saying that LTE is not supported on the HTC 8X. The specs on GSMArena say that the 8X does indeed have the following LTE capabilities.
LTE 700 MHz Class 17 / 850 / 1700 / 1900 / 2100
And, Verizon and AT&T both have LTE capable 8Xs. Does the T-Mobile 8x have LTE capabilities and they are just not going through the trouble of turning it on? Note that T-Mobile has discontinued the 8x.