I believe it supports the correct bands, the radio is just not designed for HSPA+. I could be wrong about that, but either way the i9020t Nexus S does not support T-Mobile USA's HSPA+ network. That means 3G only. I generally get 2-5 Mbps down and 0.5-1 up. Your results, of course, may vary.
TMO is working with the 1900 band. My Nexus S will show an H. This just started recently.
It showed 3G only before. Right now it's going between 3G and H.
Can't tell you the speed. But phone is switching on its own.