For this big of a wheel size, you will require at least +45mm offset. You will also have to do some custom trimming to the fenders in order to fit a 225/45/18. Some have tried it with minor rubbing at sharp turns.
Technically speaking, increase tire size beyond the "desirable" limit will reduce the performance of the car due to tire weight (moment of inertia). Until you are willing to pay more than $500/wheel to get a 15 lbs/wheel. Anything beyond that is just an added weight.