If it's that congested, they probably wouldn't be seeing great speeds on T-Mobile either. But that is somewhat uncommon... T-Mobile prioritized capacity over coverage in their network build out, so if you have a good signal, changes are your speeds are good. If your speeds are poor and your signal isn't junk, you are most likely seeing band 12, which is lower bandwidth spectrum.
There are exceptions, but for most, MetroPCS will give them the same network that T-Mobile has.