There are some on ebay, and they suggest you can fit 30mm without problems.

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PCV REPLACEMENT ECU WITH 04 WIRING LOOM STAGE1&2DNA GEL SEAT INSERT LOWERED 20mm FRONT+REAR CLUTCH ARM MOD
FORKSPRING SPACERS TALLER SCREEN THROTTLE GRIP CAM MOD DISTANZIAS 46TOOTH REAR SPRKT
SPEEDO HEALER 30MM HANDLEBAR RISERS TOPBOX MOUNT 08 XTX SORTED, ALMOST.
...SHEEPSKIN - Because a pain at the rear is not just a moany pillion.