Granted diffrent brands do vary size in mm, but i dont think is due to quality brands

 

Just had to go out & check my rear wheel liners, no scarring - Phew !. I've run Hankook V12 Evo & S1 Evo2 with no problems. Now running MP4's & all OK.

 

I'm sure another forum member on here is running 285's on the rear of his STR with no problems.

 

If you stick to the factory offset (ET60), any increase in width of the tyre is split equally in and out. Use a wheel with a lower offset, it sticks out further; good for inner clearance, but probably not great for the wheel arches, especially if you also go for a nominally wider tyre.

ET60 -> ET35 is a change of 25 mm (1") outwards.
Tyre width increase from 275 -> 305 adds another 15 mm (0.6") to the outer position of the tyre.

Do you have 40mm (1.6") clearance between the current tyre and and anything a wider tyre might contact?

Also, the wheels you mentioned (20x9 +35 5x108), would they also be used on the front. A change in offset at the back mainly affects the load on the bearings and suspension, with a knock-on effect on handling balance. However, an offset change of 25 mm at the front will certainly result in a horrible transformation of the steering