It's Accusative. Which for 'masculine-personal plural' is identical to Genitive.
I think that tere can be "efektywnych" too, not only " wydajnych".
'czy masz efektywnych pracowników' should be accepted, i agree with ma73usz
what about "skutecznych"
Makes sense to me, added.
It would seem natural to have the formal pronouns as options here, but I tried a few variants without success. Have they been added?
Not until... now.