I've done the mod and there is a noticeable gain in performance, the only drawbacks i found is that now i have a backfire on decel and cant seem to shake it but it is not as often since i put Thruxton needles in it the other day.
My only concern with this mod was getting water and road grime into the stock paper filter from the back wheel in the rain, i over come this by useing some foam for lawnmower filters but recently discovered that it didn't let enough air through to make a differance, while searching my garage for something else to use i spotted a mask that you use for when your sanding timber or spray painting , you know the little paper type with the elastic to hold it on ya head and guess what they are waterproof and let heaps of air through. Just cut it to shape and poke the dome part up into the air filter and the rest between the filter and bottom of the airbox, and "presto" a waterproof prefilter. Oh and cheap too
