I believe the l9 will do HSPA +42 so it will have fast "4G" but no LTE.
I have personally seen 26mbs down with 3mbs up with HSPA + 42 but as usual your results may vary depending on market/signal.
And yes for the metro byod you ideally want to have a device with native 1700/2100 AWS for both HSPA and LTE
The Nexus 4 is one such phone, even though LTE is not officially there.
Another will be the newer t mobile galaxy S3 SGH-T999L the older one the SGH-T999 has the HSPA + but not the LTE.
Google play edition of the Galaxy S 4 also will work well, but unfortunately not the Google play edition HTC ONE.
Thank you , so helpful . So one last question , the l9 it will not connect to Metro's 4g lte (cdma) anymore ?