People say the K&N is utter rubbish, in many ways I see where they are coming from due to its placement but hear me out............
Back in 2005 we took my girlfriends completely standard yellow Seicento MPI to Track and Road in Dagenham and did 4 runs with different induction kits. It was a cool evening, however that should have potentially run as a benefit on all four runs.
The runs were done one after the other, all conditions replicated.
The standard engine was a strong one and recorded over 59bhp stock with standard airbox.
K&N - BHP went up 3 and a bit bhp, peak torque fell 0.5 ft lb.
Focus Type Kit - dropped 1 bhp but gained 2 ft lb of torque.
GSR - BHP went up 2 bhp, torque raised 6 ft lb.
So in fact the K&N gained the most bhp although lost out in torque mildly.
TBH an induction is going to make a hairs difference of performance on this engine, so just pick what one you can for the best money / look to suit your car.
Back on total topic - best cheap way to improve the car is give it a damn good service. Fresh OE plugs, flush the oil and really get it running spot on.
If steps above have been taken add a sports exhaust backbox (as the standard is pretty restrictive) and go from there. A good backbox can typically add upto 3-4 bhp.
I am thinking of putting a K&N 57i kit on it, so is this a good invstment/ idea? And is there any other thinks i could do? 
