Your question got me curious and a good excuse to take a break from yardwork. I took a 15x7 Rota wheel fitted with Nexen 195/45/ZR15 tires from my TCS and tried them on the rear of my S2. The first problem is that the tire is too wide for the S2 wheel well so can't get the normal ride height and normal camber. This may affect my measurements.
The Rota has 4-3/4" backspace. From the edge of the rim to the nut for the lower shock mount bolt, I measure 1/2" clearance. I can't measure the clearance between the rim and the tailing arm without a lift but I suspect the same. Without seeing what rear camber is with these wheels, don't know what the clearance will be to the rear springs. I suspect there will be adequate clearance.
In my case,I believe that I can go to 5" backspace and still have adequate clearance, if I can get a tire to fit. Your experience may be different.
Joji Tokumoto
Fallbrook, Ca