Re: Speaker sizes.
dont fit speakers on the rear shelf, use the rear doors for your rear speakers (thats where they are already).
if you're going to have rear speakers it makes a lot of sense to make sure they are exactly the same as the front speakers.
sound deadening only has a benefit where it reduces road noise significantly, and in the case of 156 doors that isnt going to happen. the vast majority of road noise comes from between the door and car, and you cant put sound deadening there, all you can do is fit new seals and adjust the door position to ensure a tigh seal. the sunroof is another big culprit, but again you cant use sound deadening there either.