Although tire availability would be better for a 21", as well as tire selection, the TKC80 and other similar tires, (Karoo etc.) are not only very capable tires, but are much safer on road than many other tires designed for a 650 sized bike.
The gains converting to 21" will be minimal. Compare a 21" tire to your current 19" tire. Overall size difference is very small. (not 2") in fact, depending on exact tires used, it is less than 1" difference in diameter.
Woody's makes a conversion kit for the GS.
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