I was under an impression that if boring out to an additional 0.5mm or 1mm, the problem is that the oversize rings aren't sold separately from the pistons and have to be purchased as the part of the whole piston set.
I did a quick search on ebay and was wondering whether these rings are the separately sold oversize rings:
http://cgi.ebay.com/ebaymotors/Genuine-Suzuki-GS500-Piston-Rings-Set-0-50-Oversize-NOS_W0QQcmdZViewItemQQ_trksidZp3286Q2em20Q2el1116QQhashZitem370046609704QQitemZ370046609704QQptZMotorcyclesQ5fPartsQ5fAccessories
http://cgi.ebay.com/ebaymotors/Suzuki-GS500-Piston-Rings-Set-1-00-Oversize-NOS-Genuine_W0QQcmdZViewItemQQ_trksidZp3286Q2em20Q2el1116QQhashZitem300208565615QQitemZ300208565615QQptZMotorcyclesQ5fPartsQ5fAccessories
Both state for "Suzuki GS500EPD 1984"
Not sure what the GS500 1984 was and whether it used the same pistons as our 500 twins.