There is an eternal debate with larger wheels and performance benefit and frankly it boils down to personal opinion & taste.
With a lower profile such as 18" you are looking at less sidewall flex during cornering thereby leading to less body roll and greater or sharper response from the
suspension.
The cons is in the MI or Moment of Inertia of that rotating mass, increase in unsprung weight. Even if you go for a lighter tyre and the overall weight is the same as a 17" combo, the MI increases as the mass is distributed from the centre. Aesthetically it looks great.
The benefit with a 17" combo is less MI (if you are to switch to a lighter tyre), decrease in unsprung weight and you can possibly choose a manufacturer such as Bridgestone which makes a stiffer sidewal tyre. A decrease in unsprung weight is beneficial for the dampers and is easier on the brakes as well.
There is a benefit of a sidewall flex too. True it adds to bodyroll and negates
suspension movement to a certain degree but if the right tyre is selcted then the flex generates what is termed as the "mechanical grip" of the car. Generally speaking during cornering, aero grip of the car is greatly reduced, especially downforce and the car relies on a good mechanical grip. Mechanical grip is generally induced by softening the
suspension to a certain extent. (F1 cars is a good example of larger sidewall, to induce mechanical grip, play with tyre pressures and decrease in MI & unsprung weight).
I found the best setup on my Zed4 (with say an ideal ~50/50 weight distrbution) is to go for a staggered setup. In a staggered setup the front is stiffened (combination or one of stiffer springs, dampers or sway bar or all 3) but the wheel diameter is smaller and or the width of the rim is about an inch less. The diameter of the tyre is increased slightly than the rear say about 1/2 of an inch.
The rear wheel is either about an inch larger in diameter and or an inch larger in width. The
suspension setup is softer by the use of softer damper, spring rate and or sway bar setting.
This induces what is termed as a slight understeer and you can throttle control the rear to negate this during mid cornering.
Lot of bimmer guys go for the 18" options (18 * 8 all the way around). But i found that the Zed behaves best with 17 * 7.5 (224 45 17) up front & 17 * 8.5 (245 40 17) in the rear, with slightly less wheel offset in the rear (in crease in rear track). It's staggered and the softer sidewalls of the Contis complements the factory
suspension setup.
On my Benz the wheel diameter is an inch smaller than the rear (14" & 15") and so is the width. The Benz needs more understeer induced due to increase in natural weight distrbution towards the front and increase in rear brake bias.
I have two more sets of wheels for the bimmer (non staggered)just in case I feel like playing around but I have'nt switched to them
Rod bearings.
