i have a few questions....if i were to get a high flow cat converter does it make the car louder?? does it waste more gas? and does it void the warranty??

 

Yes,no,yes.

 

As above... yes it will be louder, although your set-up will dictate how much louder.

My gas mileage vastly improved when I changed my exhaust (although due to the foot problem it has since decreased again).

The last one I think is a little more difficult that just a yes or no answer. They must prove that your mod directly affected the part in question. My dealer told me "if you bring the car in for engine warranty work just make sure you put the cat back in".

IMO the vast majority of dealers know that Evo's rarely go un-modded and just ask you use common sense in your warranty requests.

 

I think it would be more along the lines of warantey of something like the O2 sensor or related parts.

 

sorry one more question... do i have to replace my exhaust or can i keep the stock one on??

 

You don't HAVE to do anything...

Of course if you never buy performance parts and tune it you will also NEVER be as fast as someone that did.

Notes:
Exhaust is done first as a tuning step to remove backpressure and safely allow other tuning steps like, mbc, cams, etc.
Look at the stage tuning steps on you favorite tuners web page for details (Buschur, ams, etc)

 

Was meant as a general example, but yeah what he said.