I am running 32 offset 18" x 9", which fill the fenders out nicely. However, my tires are an inch shorter than stock, so fender clearance is not an issue.
If the 20's would work with 30 offset, it would really look nice, but it would be trial and error. BPD is running 20" x 10.5" Nurburgrings on the back of his SHO (has staggered setup for looks) but not sure on offset without looking at TSW website.