OP to answer your original question I believe I am the only person who has dabbled with the largest tires to fit factory wells on a 20" rim here are some pictures of my car as it sits today on ZERO alterations to the factory suspension
the car currently sits on 275/40/20 NT-05 fronts and 315-35-20 NT-05R drag radials on the rear. this is probably the ABSOLUTE biggest you can run in our wheel wells. I'm actually currently awaiting coils so I can tighten down the dampening to keep the car from squatting and bottoming into the fenders. There's not even a fingers width of room tire-fender on all four corners.