Keep in mind, the '91-'92 Mustang's got larger inner fender liners that previous years to accommodate larger wheel's. This is another reason many have rubbing issues with earlier cars and others with later ones don't.
I've got 17x9's on the front of mine, with 255/24/17's on them and they used to rub the lower edge of my A-arm. That is of course until I motivated the edge of the A-arm inward a littler with my BFH.
Maybe hes mistaking it rubbing at full lock? I have an 89 as well and i dont rub until full lock. other wise i am ok and i am lowered. 17x8.5 93 cobra wheels 245/45/17