Mine were clogged and the MIL light was on for insufficient flow and I just cut the pipes, beat the bricks out of them, welded em back up. After looking at the brick material it was covered in black residue, they were about 75% Clogged. After removing them my truck idles better, there is a noticeable sound increase, and I would like to say it's more drivable. Not a day and night difference. But that fix worked for my 98 at 120,000 miles. And yes, I would have loved some hi-flow cats, but I was strapped for the $$$ at the time and gutting them was cheap peace of mind because I had just gotten the truck and didn't want my valves getting warped from the heat. Eventually i'd like to get some new cats when the time comes to replace the whole shabang.
If yours are not giving you problems, dont mess with them. They are designed to keep Nitrogen levels down. Yes I know the atmosphere is mostly nitrogen anyways but look at Chris said and google a little of Mexico city Smog and similar environmental freak shows in China.
Environmentally, I'd rather have no cats than a totally ruined motor. It takes a lot of energy, materials, and man power to produce one.