Don't hollow out the cats. By removing the material in the cats, you basically produce an expansion chamber that slows down exhaust flow, and then speeds up again on the other side when it reaches the normal exhaust diameter.
You can effectivly produce a standing wave, and at a certain RPM/RPMs this can cause excessive pressure. If you dynoed your car with a hollowed cat, it would likely have jagged spots where standing wave formation occurs.
Leave the ghetto mods in the ghetto. A cheap solution for a clogged cat is not a performance mod just because the cat "isn't there" any more.
As for the poster of the thread, your car will not run in any sort of respectable mannor with out a front O2 sensor in place.