Just found this document on the internet http://www.volkspage.net/technik/ssp/ssp/SSP_332.pdf which is an Audi self study module. According to it the Sportback uses an air pressure sensor in each front door to deploy the airbags. So it contains the following warning, so looks like I won't be changing to a 5 door any time soon 
Quote : Note
The airbag control system uses sensors integrated in the front doors, etc. To avoid any
impairment to the function of the side airbags, no modifications should be made to the doors
and door trims (e.g. retrofitting of loudspeakers).
Damage to the front doors can cause impairment of system functions. All work on the front
doors should be referred to a specialist workshop.
Just a warning to you all.
Paul
Quote : Note
The airbag control system uses sensors integrated in the front doors, etc. To avoid any
impairment to the function of the side airbags, no modifications should be made to the doors
and door trims (e.g. retrofitting of loudspeakers).
Damage to the front doors can cause impairment of system functions. All work on the front
doors should be referred to a specialist workshop.
Just a warning to you all.
Paul
