I dont always get fast speeds. I'm guessing at&t phones get higher priority than other phones. There is a build.prop that you can flash/edit that may or may not work. I, myself, haven't made any mods because I get higher speeds usually. Not always. I am on a 900T but using an international 9005 rom with T-Mobile modem still. It really boils down to is the risk flashing a modem or build.prop worth it. In downtown houston I rarely dip below 10mb/s on LTE and occasionally see over 20mb/s. The congestion is real bad here so it is very often I get bumped back down to hspa. Like I said, you have to weigh the risks. I am fine with how it is working now. My post probably hasn't helped you get you where you want to go but it may help you to be content with what you have or may consider flashing an international rom minus modem and see if that helps.
sent from my sm-9005.