The truth is it varies from car to car and a lot of other factors can make a difference to. Like: weather, ambient temperatures, fuel, tyres, engine condition, type of rolling road etc etc.
You've got the basic idea though as to increase performance you will be looking at needing plenum spacer, air filter and exhaust system/Cats.
If you have a good read of the dyno results thread (link below) then it will help give you some idea of how it all varies. When I had my first UpRev done with all the above mods my car yielded 295Bhp odd whereas a seemingly identical age Zed with the same mods just a few weeks later got over 300Bhp.
Link: http://www.350z-uk.com/topic/23450-dyno-results-info-thread/
As long as it makes an improvement to your car and you're happy with how it drives afterwards then the numbers really are just numbers.