under the seat is a rubber inlet snorkel that you dont need, i think it might be there in case you think your bike is a submarine.
drilling out the airbox is something supermoto riders do to improve the airbox on tuned up competition bikes, but i cant think how it would improve a low tuned bike like the XT. it also lets all sort of rubbish into the airbox. looks trick though i guess.
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dave
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