First off, there are way too many variables in raid encounters to effectively rate mastery vs haste. I wanted a controlled environment with everything the same except for mastery and haste. As far as testing, I performed 5 different sessions lasting 3 minutes each with stacking mastery. I then performed another 5 sessions stacking haste. Numbers did change obviouly, but the haste dps was always less than the mastery dps. Mastery was around 14.6k dps while haste was around 14.2k dps.
Did you had full raid buffs/debuffs on both your self and the dummy?
Also that 0.4k dps difference is well within the margin of player error and bad rng.