There are two things that seem to be commonly modified on the stock airbox. First, you can remove the two baffles on either side of the air horn, they serve primarily to reduce noise. Second, you can remove one or both of the metal screens used to straighten the airflow for the MAF. However, some people report a rough idle, vibrations, etc, when removing the screens, especially if you take out both.
Here is the DIY thread regarding the stock airbox mods: https://www.rx8club.com/series-i-do-yourself-forum-73/diy-better-flowing-intake-box-similar-rb-intake-box-130332/