Buckers

Hi, I have an A3 sportback 5 door 2009, the wheels are getting properly scrappy now and need a refurb. I'm thinking of buying a different used set, getting them refurbed and then swapping them over. I have the 17" out of the factory but I like the look of the 18" which can be picked up on eBay for a reasonable amount. Are these a straight swap? The overall tyre diameter is almost identical (due to the reduced sidewall size) so no worries there. I think though that the offset is different (56 Vs 54) - can anyone confirm? As I understand it the 18" will stick out a little more as presumably the sporty models have a slightly wider stance. Is this an issue? Does anyone know in real terms how much wider 54 will be over 56 on the same width wheel? Thanks!