Hmmm, those AKG bushings are 90D and recommended for track use only....
I'm guessing the NVH is kind of high and while stiffer is better, thinking you can also over do it. It's always hard trying sort out the manufactures literature but based on AKG's own literature polyurethane seems the better way to go for a variety of reasons.
I'm thinking I may actually give the Prothane bushings a try. I'm curious as to some of there design features but upon receiving them I'm going to test the hardness, deflection. They have a lifetime warranty and since I'm doing the install...not a huge deal if I have to replace them, but would rather not. The first time will be the hardest simply due to trying to remove the OEM bushings, the urethane should be a cake walk from that perspective.
The Powerflex bushings drawback would be the one piece rears...the deformation on install and that'd be a pain to remove.
As for a review, I'll give one in relation to stock but obviously would have no perspective on the M3 or Powerflex.