Cat-converter quesiton

That's OBDII doing the work - there's o2 sensors before and after the cat.

Hollow it out will probably set off your check engine light but getting an aftermarket cat will not.

Be careful about cheaper high-flo cats, not all of them pass e-check (if you have that in your area). You may want to look into an O2 simulator for the rear sensor. The first O2 sensor looks at the exhaust and helps to determine the air/fuel ratio, while the 2nd one is simply to verify that the cat is in proper working order. By wiring in the O2 simulator into the 2nd sensor, the car will always think it is working properly, regardless of if it is or not. Just a thought. O2 sims run about $20, but if you do have the know how to do it yourself, only a couple bucks.

thanks but
where can i find the stimulator?

I also found the related topic over wrx forum. it is very good

http://forums.nasioc.com/forums/showthread.php?s=&threadid=138177&highlight=mil

The company I bought from has a limited variety, no Toyotas, no Camrys. :

www.performancecenter.com

Go through and select your car. Scroll down once it bring up your options and it should be there. They cost about $140. Nice ones too. None of that cheap crap. And they are OEM application too. Direct fit