hmmm I think I see where the confusion is, offset is taken from centerline of the wheel so the back spacing will also change each time wheel width is increased. check this out
http://www.usacomp.com/terms.htm
backspacing is the "T dimension" so the backspacing can't be larger than 6". That means anyhting more needs to be pushed out.
Empi 8's are 5.5" wide with a +20mm offset. But this was not stock and because of the changes in suspension the allowable wheel/tire combo will change.