Re: standard airbox versus.....
Sure Alfa spend loadsa dosh developing the intake system, but at the end of the day, it's a compromise between high and low end power.
My car may have suffered a little at the bottom end (although with all the stuff done to it and replaced, it is far better than when I bought it), but I mostly have the engine buzzing at high revs, where I do notice a difference. The benefits will depend on how you drive the car and the condition of other parts ie plugs, leads, exhaust etc etc.
I do agree with the good panel filter though, I found my BMC panel filter to much better than the standard, but only after I cut the box up which suits my style of driving.