As mentioned above, the OP is ten years old.
I wonder if the bike in question ever got sold and how close it came to that $15,000 figure ?
I can accept that to some folk the bikes special history gives it some special value.
But to me it's just an XS2 with a 743cc big bore kit fitted, and I'd base my valuation on the condition of the bike, mileage etc.
Still a very desirable machine but I wouldn't pay a premium price just because it's a factory special.
As for the the "GS650" designation, rather than the more logical "GS750", I've got my own theory about that.
I'd guess that the factory just rounded up a batch of 200 XS2's straight off the production line.
Maybe their competition department did all the conversion work and then simply stamped a "G" in front of the S650 engine and frame numbers.
In a strange parrellel, I think Triumph Corp in America built the T120RT 750cc twin using a Routt big bore kit, and simply stamped a "T" on the end of the stock T120R moniker.
Apparently (suprise), due to the high value of the T120RT, there have been a number of forgeries detected.
I wonder if anyone has got hold of a "G" stamp and tried the same trick with an XS2 ?