I've got this diffuser (maybe pre-production #002). I did notice some increased stability under heavy braking. The tail end of the car doesn't wag around alot. I did have to bastardize the install to get it to fit my Motordyne Cat back (the three inch pipes with 5 inch tips are not compatible). It should fit most other exhausts though. The product extremely well finished and the hardware that comes with it is very nice. You'll see the difference more if you have the split style exhaust (MD, FI, HKS) since you wont have air getting trapped under the bumper cover.
(I had the diffuser before the wing so that assessment is accurate before adding more downforce)
At ZCON this year:
