Comparing Street tires and Street wheels to Drag Slicks and Wheels is not going to work. They are two completely different animals.
On street tires I agree with you, there is more room to deepen the backspace and move the wheel inward. My 245's are 9.7" wide (section width) on 8" wheels, with about 8" of tread width. Which means the tires overhang the wheel by about 0.85" on each side. These are 8" wide wheels with a 35mm offset and are about as close to the struts as you'd want to go. And the backspace is about 5.75", which is deeper than stock.
However with slicks, there is no more room. I have about 3/8" to 1/2" between my coilover and the edge of the slick at the closest point, that's with the narrower 8" Hoosier slicks. With the M&H 8.5" slicks its is about 1/8" to 1/4" (the M&H's also have a considerably larger section width). There just simply isn't any more room to deepen the backspace. So the backspace must remain the same, as the wheel width increases (to accomodate larger slicks).
Here's some graphics to help explain the problem:
As you can see there isn't alot of room to start with between the slick and the strut. If you try to deepen the backspace then the slicks will start rubbing. The section width on the M&H 8.5x26 tires is 10.5" meaning they overhang the 8" wide wheels by 1.25" on each side.
You can only deepen the backspace so far before the tire starts rubbing on the strut. And when running slicks, it's even worse because of how wide the slicks are in relation to the wheel.
