Ian,i have fitted the JR filter to my new 9-3ss Aero and yes it does improve the initial throttle response but the downside for me is i can hear the induction noise an awfull lot more than with the standard item.
Its staying in the car though as i just turn the stereo up a bit more
Try Elkparts for the filter,i found them to be the cheapest.
I've had the JR filter in my 9000 for a year now. Not more noisy and yes a bit more responsive at higher revs/speeds. I'd let it go for 10k miles without cleaning, and there was a definite improvement afterwards !
Oops sorry, I've just noticed you're talking about the airbox as well, I've just got the filter.
I am running the JR filter with the induction kit on my 9000 2.3T and I have noticed a big increase in induction noise, performance wise, I cant say if it alone makes a difference, but, along with the other mods I have then I am sure it does make a difference