You can run any US carriers TouchWiz rom on the T-Mobile phone apparently.
See these instructions
. Also a thread in the T-Mobile forum
I haven't tried this yet, but I'm going to give the Verizon Hyperdrive rom a try later. From what I gather, just flash the other carriers rom and one of the kernels made for the T-Mobile SGH-M919.
DO NOT FLASH THE i9500 ROMS -They may brick your device.
Like I said, I haven't tried yet, so you're on your own.