T-Mobile will be selling the iPhone 5 starting April 12th, does that mean that us current T-Mobile iPhone 5 users will start getting 4G LTE coverage? Or will the iPhones sold in April 12th from T-Mobile have a different bandwidth to support existing 4G LTE T-Mobile bandwidths?
Will T-Mobile have 4G speeds for iPhone 5?
If you have the iPhone 5 model A1428 (North American GSM model) then yes you will be able to get LTE. If you have this model iPhone and you have it jailbroken you can get LTE functionality right now without having to lose your jailbreak. Click the link below to my article and it will give you step by step detailed instructions on how to do this. Hope this helps 
http://leimobile.com/how-to-enable-lte-on-iphone-5-for-t-mobile/
http://leimobile.com/how-to-enable-lte-on-iphone-5-for-t-mobile/
All of the other posts are wrong, T-Mobile stated they aren't going to be rolling out LTE until next year because they are still expanding their 4G and since they recently acquired Metro PCS which is CDMA based it will take time to convert that to T-Mobiles network which is HSPA+ and since the iPhone 5 is only LTE and not 4G T-Mobile users with an iPhone 5 will be stuck on 3G and by next year when T-Mobile starts rolling out LTE Verizon, Sprint and AT&T will already have full nationwide LTE coverage and possibly already rolling out a faster network so I don't recommend T-Mobile besides their LTE will be the slowest one compared to Verizon, AT&T or Sprint.