tmobile sucks in terms of data speed i feel.
Actually, it depends. A data test done last year in the top (major) US cities showed the T-Mobile had the fastest 3G (both upload and download) and fastest
consistent 4G. (Att had faster upload 4G, but slower download. Verizon had faster download 4G, but slower upload).However, in more rural areas, and much of central US, T-Mobile doesn't really have the signal to prove whether they have good speed or not. Also, T-Mobile uses different bands/frequencies for their 4G, and in most of central America, their 3G, due to ATT owning most the spectrum, and not even all their own phones they sell support such odd bands/frequencies (such as the Lumia 810, which according to GSM arena, doesn't support the 1700 MHz frequency). So speed may vary by handset and location, but where T-mo is at its best, it is the best (in speed)
Though it will most likely get better. After T-Mobile refarms some of the Metro PCS spectrum (and some of the old 2G spectrum) it will hopefully get better.
Ultimately moved to LTE, I would imagine, so it may be a slow process over 2 years.
In an article I read, T-Mobile explained that at first they wont do anything, but they will slowly move people over to T-Mobile if I remember the article correctly, and require them to purchase new devices. and I think it was a 2 - 4 year timespan. Though don't quote me on this part because I may be misremembering the article.