First off... you cannot use target dummy tests as a reliable source of information. To give you an idea, if you wanted to know if you had gained or lost 100 DPS, you would have to hit a target dummy for 100 hours. During that 100 hours, assuming you were doing 450 seconds trials, you would see a range of DPS values that varied by 4000 DPS.
Second, I play a shadow priest, and I use Mr. Robot. My DPS has never gone down by reforging how Mr. Robot suggests. To test your DPS, you should really be using something like SimC, not a target dummy.
Third, a two-handed staff will never be better unless you don't have a good offhand to go with the one-hander.
You should post up your character realm/name and the different sets of gear you are testing - it would be very easy to quickly verify which gear is actually best for you using some simulations.