Veon

Hello, I have 2010 s550 4matic with sport package, got it with some ugly aftermarket wheels so I bought 20 inch amg staggered rims and now I'm trying to figure out tires. I know cause it is 4matic that it has to be in 3% diametar difference front to back. I was looking for 19 inch oem wheels but they are hard to find so end up geting 20inch. I am worried now that ride is going to be too stiff with 20 inch wheels so question is what is the biggest tire size I can install? I was thinking 255/40/20 front and 285/35/20 back? Now temporary on the car are audi's peeler wheels 265/40/20 and front tires are rubbing (on upper control arm I think). I saw somewhere on forum that someone is running 255/40/20 on the front but that was s65 amg, not 4matic as mine, is there a difference in front setup?