You should get better than that, see my post above, depending on your signal strength and the speed of network you can find. I usually get around 1.4mbps on H.
Did you test this when on H data connection, as that is the fastest? How often do you get H? These things will affect your speed.
I'm pretty sure T-mobile shouldn't cap your speed i never usally have problems streaming HQ youtube vids.
Are you sure you haven't exceeded any dataimits (if you have any)?