If the cat is leaking? Then have it repaired or replaced. That simple.
Main reason many people get confused with? The computer, it's software/hardware, everything is built around the existing emission control devices. Yeah, removing the cat "can" net lower back pressure--But also consider that a working cat creates a very small amount of back pressure in the first place!! While some may disagree (mainly on 'Ricer' sites). That small amount of back pressure? Does not have much, if any effect on how the car/truck drives. In fact in some cases, the O2 sensor and the computer are configured to expect that! So in essence, removing the converter and dropping in a test pipe? Can make your performance actually WORSE!
In fact, the only reason to EVER run a test pipe, in place of the converter? Is to test for a clogged converter. (one where you can do a visual and it looks fine, however the carbon buildup could be several inches into the honeycomb material used)
Just my .02cents..
S-