I recently had my sti ppp have a spot of sound proofing at Noisekiller as I use it as a daily driver and wanted to enjoy my sound system and have handsfree conversations without too much background noise. This is just a small report on the effects.

Plus

1) Reduced the outside noise considerably could even hear it from turning on the engine (did not have under bonnet sound proofing)
2) Sound inside the car a lot better bass improved from sub in boot and highs etc heard much better.
3) Can have conversations much easier in the car.
4) Biggy - The car feels like it has had new suspension the road vibrations are significantly reduced and feels a lot more smooth especially on some of the crappy 'B' roads I go down (on purpose).
5) Biggy - I feel less fatiged I actually come out feeling a little more refreshed especially from a good run on a few twisty roads
6) You can still hear the engine note and exhaust although it is reduced.

Negative

1) Burble noise reduced
2) Added weight of 10 - 18kg (guessing) but I can't notice any speed difference

Took approximately 2 hrs to fit at the factory in Manchester and cost £230 fitted or I think £120+ for the kit to be sent.

I'm thinking of going down the WRC350 route so wanted something that could help reduce any noise that would come from the cold air induction. But like I said the vibration absorbing was totally unexpected and just lets the car feel a lot smoother to drive.

Hope this helps somebody out there