Just fitted a set of 18'' Monza reps from Rochford tyres. they are 8x18'' ET 45 offset fitted with 225/40/18's. I have noticed that the wheels sit out a little further in the arch but looks like they are filling them a little more to me. (I used to have an S3 and a Mk4 GTi Anni, everyone put 10mm hubcentric spacers on these cars to give cars a wider chunkier look.)
Correct me if I am wrong but ET51 wheels are only 6mm different, so I guess my whells are sitting out a further 6mm each than before. Don't see any harm in that.
Not really noticed any change in handling or steering to be honest , car looks much better on the 18's though!
Finish and quality of wheels is excellent, most will not be able to tell they are reps.
As for the weight issue, they did seem fractionally heavier that the 17's that came off, but the new alloys are wider and bigger, so as expected really.
Don't think it will make as much difference to the car as the holiday weight I am still carrying!