If you want to increase the Torque on you engine get a panel filter like a K&N / BMC.
If you want a better throttle response and more BHP, then go for a cone filter
Thats the theory, i have a k&n panel filter on my 146 1.7, and just got it run on a rolling road. it is now running 141HP and 118nm of torque suppoesd to be 129HP.
The BMC cone filters in the Alfaowner shop sound lovely when you stick your foot e. supposed to be 130HP down. [img]smilies/drool.gif[/img]
As for insurance, i dont see any need to tell them, i havent. not sure if that would make any difference. if i have an accident, and they querie it, then i will say it was on the car when i bought it, and thought is was standad.