I just installed Samco Silicone Hoses kit,upgrade CBV, ST stg 2, MBC ,IPD spark plug cabel,MSD coil kit and a prosport boost gauge, K&N drop panel. and I have to say WOW !. My car has never felt this good . the power comes in sooo nice. Iam only running 15 psi for now as i still have a stock down pipe and exhaust.and what really surprise me my mpg whent from 17.2 to 21.and is not like the car needed a tune up because i had alredy done a full tune up 2 month. over all very happy. now i got a Q. can i do a 3" DP and keep the stock cat back and get any gains . I really dont whant to make the car any louder than what it is. as this is my D.D
some pic of the gauge is white when the lights are off and green when i turn on my headlights.

 

3" downpipe would work great, but there would be restrictions from the stock mufflers. You can always get 3" downpipe with an aftermarket muffler and a resinator. It will only be slightly louder than stock.