I have been using my i545 with a T-Mobile sim in the Seattle area for 2 weeks now. I was using the hyperdrive 9.1 ROM and a couple days ago switched to the Eclypse ROM. Both required adding the APN and neither giving LTE. There is another thread where a gentleman is attempting to shoehorn in a different radio but so far has only succeeded in bricking his phone, but I am holding out hope.
I have noticed that with hyperdrive the icons showed 3g 4g and HSPCA+ but on eclipse I only get the 3g icon for hspca+ and no icon for 4g or 3g on eclipse, which doesn't really bother me much but something I have noticed.
I have had zero mms sms issues i keep hearing about. I use the 4.2 AOSP messaging apk. I should note that my wife has a note 2 from verizon also running a tmobile sim and she had issues until i added the mms proxy info in the apn. Now she can send fine also. (i also had to delete the setup.apk to get rid of the sim notification on her pgone but not mine.)
One thing I am having an issue with is when I cycle the phone on and off. When it turns back on I get a notification that all calls are being forwarded to a 1800 number but when I click the notification it thinks for a bit then disables all 3 settings and the phone works fine.
I hope someone smarter than myself can figure this out. Our radios are supposedly able to pick up and frequency, so in theory it is possible. I tried to flash a 9500 ROM in hopes the radio settings would work, but was rewarded with a bricked phone and joy of an oden back up.