I think you might find this an interesting thread from the UK site.

http://rx8ownersclub.co.uk/forum/viewtopic.php?t=1969

It basically states that the guy has had a custom built Cat Back exhaust + K&N Typhoon (developed on his car), and has benefitted from increased performance.

The interesting point is that although the car has not been dyno'd with the intake & exhaust, the prodrive instructors on the driver training day (free for the 1st 1000 orders in the UK) all noticed the extra grunt (& mph).

These guys should know there stuff as they are test/development drivers!