To be honest the FEE worked exactly as planned from the get-go. It was just that they had spent so much time refining it that when the season started, the FEE had no more upgrade potential.
It's probably this that actually got them thinking, "let's not faff around with the funky stuff, and let's just concentrate on getting a good basic chassis with good balance, good drivability, and good, efficient downforce" - leading them to just plop on neutral exhausts and focus on the basic chassis.