This is a very good question, one that will be difficult to get a straight answer for.
The reason I say that is because from my understanding metro's LTE is on the AWS band, which is the same as t mo's, however in a few markets metro does not have any aws spectrum so those markets they used lte on the pcs band.
So to go on with the question, metro phones currently use aws lte but tmo has not rolled out lte in every market yet. so if you have a gsm phone that has lte on the aws band and a metro sim you should pick up lte no problem, metros own lte, but it will depend on how metro doles out the sims.