The XS1100 Special had the leading axle, the standards had the regular fork. The standard fender will bolt right on the XS650 forks but as mentioned it doesn't have the heavy bracing. The XS1100 had a 37 mm tube and as such was much stiffer and didn't need the extra bracing.
I know the XJ bikes had 36 mm forks. MY XJ650 has a regular fork. Air assisted but had the axle under the forks. The XJ750 Maxum had the leading axle fork and was 36 mm.
My friend has an XJ750 Maxum. His forks are all pitted and chew up seals. I put my XJ650 forks on his XJ750 by just swapping the forks. I had to use my XS1100 fender because of the regular axle vs the leading axle fenders are different.
He says it handles much better. The XJ650 forks shortened the wheel base a bit and decreased the trail a bit as well, this quickened the steering.
Leo