Briefly,There is a difference between flow and volume. Go to large (or to open) you lose volume (amount of exhaust that passes through) once the volume drops you starts introducing resistance (backpressure).You can only go so big before performance gets effected negativly again. There is a margin that you can run for optimal performance. Being you have a stock engine, your best bet is a high flow cat with an aftermarket exhaust system.

Exhaust gases expand as they cool, going to large in piping diameter (or gutting a cat or removing one) the gases cool and expand...I wish I had more time to get into this and explain it better but i'm running out the door as we speak.
point being your car is stock, and won't benifit from a test pipe.