What is the difference between a cold air intake like this one Eurosport Tuning: Ultra-Flo Air Intake System

and a simply fitting a cone performance filter like a K&N.
What size cone would I need on a stock 2001 S40 1.9T ?

 

Your car has a CAI already by factory while the one on the Eurosport site is just an open cone filter with "heat shield".Stick with a performance panel filter.Its a turbo engine will benefit much more from having cooler air.Every 7-11F colder air will give you 1% HP gain.Now if you replace the OEM CAI with an open cone or even with the one from Eurosport with the "heat shield" you are going the opposite direction.

 

What about increasing the turbo noise? Can that be done with stock ?

 

What do you mean turbo noise? I'm assuming you are referring to the BOV, I could be wrong, but i dont think you can get the "turbo noise" with stock components, but I know HKS makes a nice one. My roommate had one on his stage 2 wrx and it sounded amazing every time i would shift.

 

He refers to the sound coming from the airbox.
The thing is the typical whistling noise will come only with an open exhaust the filter will not do that for ya.Now if you wanna listen to the sucktion and when you left off the paaahhhh then all you do is to take the foam out from the inside of the box and use a performance panel filter..
IMO the BOV is strictly for the manual it will not be cool on an automatic.