SRGTD has given above a very comprehensive account above of all the factors to think about when looking at an alternative set of wheels.
BUT I am still in limbo with a choice of winter wheels for my forthcoming
Carbon Edition.
It comes with 19s but I really want to go to 18s (painted not diamond cut...winter use!)
Now Carbonara has already kindly pointed out that Team Dynamics pro race 18X8,et 40/45 will fit,the issue being the INSIDE clearance to go over the larger Brembo brakes.
Fine,but everyone likes a bargain and it's possible to buy a set of Wolfrace pro lite,same size and offset for a lot cheaper,only about £400 a set.
Both of these types of wheels are a similar design and are a semi light weight wheel,so you might assume that the inside clearance would be much the same,but I've tried two national alloy wheel companies so far...and they haven't a clue.
So the other important dimension,in some cases, is inside clearance diameter,which nobody states!!!