I found that a shorter diffuser, working together with a high downforce rear wing could replicate a double airfoil wing principle: the rear wing helps the diffuser work better (it already happens with the extended diffuser, but in this new case efficiency has greater advantages than max downforce). I am sure that I could reach df/dr 7 with another month of development (about the same of 2017: consider that my 2017 car, with the 2018 rules and no optimization, lost most of the efficency, from 7.0 to 5.2 in August... I could not imagine to recover al the performance).
This not the only improvement: the other one is partially introduced and visible in the car I have submitted (cooling ducts with internal flow guides). It not only improves cooling, but also helps with front df and drug reduction.