IMPORTANT UPDATE: Docomo users beware!!!! This and all subsequent CM6 ROMs as of 30 Nov 2010 contain the mopera.net APN, which may be set by default adding a 4200 yen charge to your bill!!!
It only took a few hours after the proper people got ahold of the T-Mobile official MT3G Froyo update for that build (FRG83) to to be incorporated into CM6. To left (in English) is the phone information from the hybrid T-Mo/CM6 ROM. To the right is the CM6 nightly from 10/19/2010 (209). I did indeed notice a speed increase with the official ROM but was missing a number of the CM6 features.
To try a CM6 nightly, use recovery to back up your current CM6 install and flash the zip file containing the nightly There is no need to wipe, and you won't lose any apps or data at all.
If you installed the hybrid ROM, first wipe (including cache), then restore the nandroid backup of CM6, and finally flash the nightly over the restored CM6 ROM.
Keep in mind that nightlies are experimental builds. Some work great. Other much less so. For me and a number of people, 209 is working fine. Nightly build 208, the last one prior to incorporation of the official ROM, was reportedly very very slow. When 210 is released, it could be better, the same, or worse.
If you have trouble with any experimental build, it will probably be best to go back to the stable release. Or skip to the end of the discussion of nightlies to try and figure out what the was the last one that was reasonably stable.
Nightly builds can be downloaded here.