Are you using Ultrafire 3000mah? I ask because when I use them in my TD15X the output drops slightly because the battery can't supply enough juice.
For instance with the TD15X and a Redilast 3000mah I can do a ceiling bounce compared to my Eagletac T20C2 XML and the TD15X is brighter however if I remove the Redilast and put in the Ultrafire 3000 then the T20C2 is slightly brighter. This has been a common occurrence with any of my high output high drain lights when using UF 3000mah.