Efficient Saurians still appear as grade advancements. I got one on my alt a week ago when he was in the lower ranks.
Yes, the trait stacks giving you more power the more you have. Yes, you have to have them stationed in a Boff slot to get the benefit.
Besides the Saurians, you can also get a unique Efficient Borg Boff from the C-Store. I currently have 3 Efficients - 2 Saur and 1 Borg.
My shield and engine power settings have each risen from 60 to 63. The base on both is 25. Weapon and Aux are unaffected since one is maxed and the other is as high as it can go on my Star Cruiser depending on what I'm doing.
If you're in a crusier, you should also have several of you powers see good increases as well. Its nice being able to get a few more power points out of EPtX (as well as Engy captain powers).