First some clarification:
E= 2G EDGE, 3G= Regular 3G, H=HSPA+, (what T-Mobile USA is calling 4G), 4G LTE = 4G LTE.
T-Mobile 3G has theoretical speeds of ~7Mbps, HSPA+ has theoretical speeds of ~42Mbps and T-Mobile's upcoming 4G LTE will have theoretical speeds of ~100Mbps.
I am assuming that you did your speed test when on H right? Lumia 920 has two H icons, H and H+ ??
Second the awesome news that nobody is discussing yet is that, not only Lumia 920 is capable of delivering LTE at 100Mbps right out of the box but this Developer model (and the Rogers model) supporting LTE frequencies at 1700/2100.
These are the frequencies T-Mobile is launching its 100Mbps Network next year, meaning that this phone will be able to work with T-Mobiles LTE immediately as soon as they turn it on! On top of that that, these are also the frequencies that Metro PCS is already running its LTE network right now, also meaning that once the merger is complete wherever Metro PCS has LTE, it will become T-Mobile's LTE as well and this Lumia 920 WILL be able to work on!
AWESOME!!