there are many tasteful parts for the car, some have claimed aerodynamic properties on the track, others dont. The wald kit is a bit garish for me and is not track oriented, i dont think they claim to add any aerodynamic benefits.
it wouldn't be hard to improve on nissans rear diffuser! the rear is not a masterpiece of aerodynamic design, and it is accepted that the front splitter isnt the most aggressive design for downforce
on the other hand there are some 'proven' front and rear diffuser combinations with added benefits. unfortunately some of the really great front diffusers with brake ducting and dry carbon fibre construction are frightfully expensive. nissan had to find the right balance between acceptable styling and aerodynamic properties. otherwise we wouldn't see any mods on track set ups relative to mass produced cars
having said all that I won't be adding aftermarket kits to my car any time soon, id rather spend the money for now on tyres and brakes so I can have more fun.