E_Pho_Thirty

I know its a direct bolt up, my question is will having the 20mm in the rear but not upgrading the front sway cause any issues?

khomer2

Why only one?
Changing the rear ONLY... is going to effect the relative front to rear roll stiffness which is highly probable to alter the understeer/ oversteer characteristics of your w210.
I assume you do not have the sport option on your E430. As to whether it's a benefit or a detriment, you'll be the judge when cornering.