Two things. First, raid buffs. If you don't have them when you test, it doesn't necessarily extend to a raid situation (unless in the raid situation you don't have those buffs, in which case you should try to get them). Secondly, reforging options will not cause a huge (read: 1200) dps variation. If I were to guess it would be a rotation issue causing the disparity.
I ran it by Mew (higher than 378 though) and the haste getup had about a 200 dps advantage over the mastery set up while self-buffed. That would enforce the idea that your rotation is off slightly.
What is your armory? With that we can look further into the issue.