Williams F1 started the season with high hopes after it was so proud of developing a new extremely low gearbox. This allowed the designers to clean up airflow at the central rear part of the car. While early performance measures during winter tests were still promising, a real race weekend quickly showed the car's lack of speed.
By that time, Williams were already working hard on a now exhaust solution. When Red Bull showed up at testing with their exhausts blowing around the diffuser, just inside of the rear wheels, Williams - and pretty much every team - started to investigate the idea and decided to copy Red Bull's solution as early as it could. The new system therefore routes the exhaust pipes towards the outside of the diffuser, while the floor is cut out at the exhaust exit.
It is however well known that exhaust flow is something particularly difficult to simulate, even in a wind tunnel as gasflow is varying constantly. The team tried the solution or Rubens' car but eventually went for their previous design as there were too many uncertainties. With the team planning another major rear end upgrade for Turkey or Spain, it even remains to be seen if the tested version will be run again. At least the team now have some data to look at.