Unless you're getting it tuned to suit, chasing any sort of measurable economy improvement with minor mods is a fool's errand imho
Yes, better airflow may well assist slightly in that department but not something you would expect to be noticeable.
Also, any additional power is just begging to be used.....with obvious results in economy.
Holesaw through the bottom of the airbox is the best value cheap/free mod in a T, assume it would 's the same in an N/A
What sort of 'real' data are you looking for? X.XXkw for a flat panel filter on a 2005 BF N/A XR6 with a catback exhaust...?
Edit
beat me keef