I've been using this approach to since it came out about a year ago. (Though I did take a hiatus from STO for about 7 of those months.)
Depending which DOffs I have available at any give time I run the mission, I get anywhere from 20-25% or so for a Purple. I haven't keep a specific log, but I'm certain I've run the missions a couple dozen times and have received a purple Mk XII from a half-dozen of those attempts and in all cases it was something of no value to any of my toons (or the Exchange), so I vendored it. I did get a Mk XII Disruptor and a Mk XII Plasma boosing Tac consoles once each. And those were nice. Every other attempt came back with a worthless Green console. And not even a "good" green.
I don't go out of my way for these missions anymore. If I get the artifact I sell on the exchange and I save up for the consoles I want. Less frustration, but more direct/easier that way.
When XP earnings during a Double-XP Weekend still feel like I'm underperforming,
there's something terribly wrong with the reward system...
You can find/contact me in game as @PatricianVetinari. RIP Terry Pratchett. Original Join Date: Feb 2010.