This topic can have a whole dissertation written on it. It is a bit more complicated than simply "bigger rims for bigger tire" or " taller sidewall = more flex = better grip". Wheel/tire is part of the suspension. The whole system must be engineered properly for it to yield optimum performance.
Stated simply... 20" wheels
can work well. Some sports cars have OEM 20's, needed to clear very large brakes. C8 Corvette's carbon fiber wheels over huge carbon ceramic brake is a good example.
Ideally, for optimum performance with the lightest weight and lowest rotational inertia, wheels should be no larger than necessary to clear the brakes. Stinger's front Brembo brakes easily fit under just about any 18" wheel. The smaller Brembo in the rear would fit under 17" wheels. Some smart Stinger drag racers do fit big fat 17" drag radial on their rear axle. That is, of course, optimized for straight-line 1/4-mile. Track/AutoX applications and serious canyon carving work better with all 18".
Also, the suspension tuning needs to be matched to the wheels/tires. Problem with the Stinger is that the stock suspension is wayyy underdamped, no doubt meant to give the Stinger a supply enough ride with the OEM 19" wheels and low-profile tires. The consequence is that - when the pace quickens - the soft rebound damping has trouble pressing those low-profile tires (with very little compliance) against the pavement. So the heavy wheel/tire assembly bounces and pogo's over road undulations, breaking traction and losing grip - quite often constantly swapping sides and upsetting the chassis.
Lighter weight wheels HELPS with this. Even larger diameter wheels with even lower-profile tires (less tire compliance) DOES NOT.
You know what does help? Swapping to wider 18" wheels, so you can run much wider tires while maintaining the same aspect ratio. This preserves enough tire compliance to help keep the tread pressed against the tarmac.
You mentioned Eibach pro springs. Yes, they are roughly 10-15% higher spring rate than OEM springs, so they do help... albeit only to a limited degree, because you still have the underdamped OEM shocks trying to control even more massive wheel/tire assemblies. Unfortunately, nobody sells uprated aftermarket adjustable shocks for the Stinger/
G70 platform. The only way to get better damping is to go coilovers. Unless, of course, yours has electronic suspension control with Mando controller, which might allow you to firm up the dampers sufficiently. No first hand experiences here, so somebody else can chime in on that.
Would I go 19" or larger? Sure... but only if I do a Big Brake Kit that needs the added clearance.
