Now that I think about it, the Subaru offsets that are over 50mm are probably trying to take advantage of the narrow wheels that are being used. If Subaru, for example, spec'd a 6 inch wide wheel...then they might choose to use a 60mm offset. Why? Several reasons. First, steering feel is improved as the scrub radius is decrease (or eliminated). Second, toe change under power or braking is decreased (or eliminated).
The question really should be "How can the aftermarket build struts that increase clearance from the strut body and the inside lip of the rim?".