I think that most of us agree GSR or CDA is the way forward, as KC says Peter has not finished work on the GSR kit yet hence the reason you don't see any on the b - once they are available I will be visiting Peter for one.
I noticed the difference when I fitted the green filter kit, I don't have graphs and stats to back it up but once on the move there was a bit more oomph, especially at 3.5k rpm or above...................but oh that sound, a real throaty growl that is often mistaken for the exhaust!
Each to their own on this one, at what point do you reach an 'adequate' amount of horses under the bonnet of the b? (unless you are building a sprint or track car)....
Once you start cresting 150 bhp+ how do you get the power on the floor? The b does not have a lsd and it's 'twitchy' enough in standard form, also it already has a taste for front tyres (I have just replaced mine after a mere 5k miles!!). I still think that 150BHP-ish plus some good handling modifications makes for a faster (and better balanced) car than a standard car with bag loads of horses...?
Maybe I have the wrong end of the stick here but we are all enthusiasts who get a kit out of personalising our car's, making them sound nicer and a little bit faster, we are not professional race teams where the additional couple of BHP does make a difference on the track! As long as we are not doing anything detrimental to the car's it's great to see everyone doing there own thing - at the Nottingham meeting no 2 car's were the same!
