done a bit more research about rns-e in older audi models:
on the rns-e models, only the a6 unit has a gala input - all the other models use gala over the can-bus
the older a3 and b5 a4 doesnt put gala over the can-bus, so you can only successfully integrate an a6 model into the a3/s3 with any ease.
the a6 model seems to be the right height top to bottom as well, leaving only the left and right sides needing cutting to get a good fit.
models with can-bus (post facelift) can use the can-bus for radio/nav display on the dis anyway and also have canbus signals for the lights being on and the ignition on - this means you only have to connect the can-bus from the back of the navi to the intruments ( you may already have these wires, particularly with symphony II radios or older satnav plus) and the existing radio gala wire to the rns-e v wire and the whole thing integrates nicely.