my personal experience with K&N have been improved induction noise but unfortunately reduced BHP, admittedly not on a Fiat.

I fitted a K&N typhoon crossover pipe on my MX5 with before and after Dyno testing. Lost about 6BHP.

Well thats the bmc fitted! Just awaiting the ally pipe got a spare section but its about 5mm short on diameter would probs work but hey i paid for it so im waiting haha got it running and warmed up then took it for a little blast down a road near mine shocked a merc driver but although not really any noticeable noise a much quicker response is there and it feels really good i could see it improving mpg around town as not having to push it as hard. All in all i rate the bmc so if anyone is looking i definatly put a good word in for it also got a really cool sticker which i placed on the air box haha