Induction kit is basically a tube that is 'tuned' for the engine. I'll run through some background to hopefully make things clearer
:
Long induction passage (like a long tube say) = +torque, -horsepower. To an extent.
Short induction passage (say, a cone filter sitting directly on top of the throttle body = -torque, +horsepower. To an extent.
I say 'to an extent', as, with a filter sitting directly on top of the throttle body, you would need to rev the car to around 12,000rpm to be able to extract max horsepower. Therefore, for the rev range that we use on these cars (which is nowhere near 12,000rpm!), a longer induction passage is required to strike a good balance between HP (horsepower) and torque... an induction kit. The cone type filter on the top of the TB (throttle body) is not an induction kit, it is an air filter. There is an air filter on the end of an induction kit also.
There is also another downside to having the filter on top of the TB which has already been said. Heat rises and, being situated right on top of the engine, lots of hot air will inducted.
Hot air = less dense (less oxygen for a given area) = less power
Cold air = more dense (more oxygen for a given area) = more power
You need fuel, oxygen and a spark for an engine to run. More oxygen is therefore a good thing
.
So, another positive for an induction kit is that it situates the air filter in a place where there will be cooler air.
Now, arguably the best kit out there for the the 1.1 cinq is the GSR kit. It retails at £100 delivered, and it will give you a nice roar too. It can be found here:
http://www.gsr-uk.co.uk/engineering/pages/productsbycar/fiat/fiat_cinq.htm
Bottom of the page. Gosh I sound like a talking advert for GSR here, I'm merely a customer, and not paid to say this!
Alternatively you can try and make your own, I think I saw one in the classifieds not too long ago. Or on ebay the focus induction kits fit, I think. You would have to clarify with another member first.
If your just after noise and not too fussed with power increase/loss, then go for the filter option, it won't disappoint in the noise department!
HTH