This has really caught my interest now, so I've looked into the relevant details:
Forester (03-08)
PCD 5x100
Centre bore 56.1
Offset 48
Mounting Nut/lug 12x1,25
Tyre sizes 205/70/15, 215/60/16, 215/55/17, 225/45/18, 235/35/19
Impreza (01-07)
only difference is Offset of 55
Impreza WRX STi (00-05)
only difference is Offset of 53
So the only question now is what does in mean in easy to understand terms to go from an offset of 48 to 53 or 55?