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A certain amount of sidewall flex is necessary to generate maximum grip. This is called "Slip Angle". The fact that there is less sidewall flex on a tire does not necessarily equate to better cornering grip and as a matter of fact there is a point where if the sidewall is too stiff cornering grip will suffer.

I would not characterize " there is very good evidence that the 18s outperform the 19s." as a blanket statement, but for what it is worth at least some (if not all) of the racing versions of the M3 use 18" rims. It would seem to me that if 19s were faster they would use them.



Here are the specs of the Rahal Letteman racing version of the M3.

Note that it has 18" Rims.

Technical Specifications of the M3 Race Version.

Weight 2,745 lbs./1245 kg
Tank capacity: 23.7 gallons/90 Liters
Chassis/body: Unitary construction steel body with welded safety cell made of extremely rigid precision steel tubing; safety fuel tank in CRP sandwich tray; pneumatic four-stamp jack system
Aerodynamics: Front fenders, rear apron, hood, roof, trunk lid, rear wing, front wings, and flared rear wheel arches in CRP
Transmission: Carbon fiber clutch with hydraulic central slave cylinder;

6-speed sequential racing transmission with straight-cut, unsynchronized gears;

additional oil/air cooler; quick shift system with ignition cut-out controlled by shifting force; mechanical limited slip differential with additional oil/air cooler

Front axle: Based on production version, with increased wheel caster angle, enlarged track width and enhanced wheel camber; five-way adjustable shock absorbers; tubular stabilizer bar
Rear axle: Based on production version, with enlarged track width and enhanced wheel camber; five-way adjustable shock absorbers, tubular stabilizer bar
Front brake system: Six-piston aluminum brake calipers, inner-vented grey-cast iron brake disks 15.0 in./380mm in diameter
Rear brake system: Four-piston aluminum brake calipers, grey-cast iron brake disk, 13.1 in./332mm in diameter
Steering: Rack and pinion steering with electro-hydraulic power
Wheels: Aluminum wheels, 18 inches