I'm not sure if it's coincidence but when I first replaced my Nexus 5 with the M8 last week, the LTE speeds on my M8 were much slower; I'm talking borderline 2G/3G when 4G is indicated. This wasn't the case everywhere but around the areas I usually use my device, I noticed the M8 was generally considerably slower than my Nexus 5.
Then I got S-Off and flashed the GPE RUU, changed the CID and technically have a fully functioning GPE. My speed tests since have been better than my Nexus 5. Highest I've seen on that was maybe 25Mbps down, 20Mbps up. On the GPE M8 on T-Mobile, I've often seen 50Mbps, 20Mbps up.
Has T-Mobile possibly improved service in the NYC area the past few days or is it a combination of different hardware/software? Perhaps the radio?