Hi, I'm using K&N filter about 1 year (and about 3500 miles), and here are results:
I had to upgrade to .88 jet (stock was .78, I tried .82 and it didn't work) and do some minor carburetor adjustments, scooter is much much louder, but loudness decreases with time (as filter gets clogged from dust), filter is screwed directly on carburetor.
K&N filter increased acceleration and noise level, top speed wasn't affected by mod.
It's cheap mod and it helps, stock air filter has a very low air flow when compared to K&N
Scoot is
agility 50, '07, (6.2g rollers, Vespa ET4 variator belt, K&N with .88 main jet, unrestricted CDI, scooter has done 12000 miles)