O2 sensor

If I remove the cat.conv. on my 97 Wrangler and install a test pipe. What effect would I have on the engines performace? And, would I have any warning lights on the dash as the CatC. has a O2 sensor attached?  

If you remove an O2 sensor your Jeeps OBDII system will throw a code, and your engine may suffer from a loss of performance and gas mileage since the engine's computer gets info on how to run from the exhaust gasses. Removal of a catalytic converter from your exhaust system may allow a tiny increase in horsepower at the top of your engines RPM range, but it will also probably cause a drop in your max torque at a lower RPM (Which is generally more important than high RPM HP to a Jeep being used off-road, or driven on-road the way most people drive on road (not like a race car at 4,000-6,000 RPM constantly). This loss in torque will probably will be more noticeable than any gain in high RPM HP).  Modern catalytic converters flow pretty well and what back pressure they create helps with torque development.

I wouldn't remove it if I were you If not just for the reasons above, it is also illegal in many areas around the US and can carry stiff fines if you are caught. It also can make selling your vehicle more difficult as the new owner wont want to have to pay for the cat to be replaced or worry about any emissions standards they may have to meet.

If its rattling then it may need to be replaced In which case you can either have it replaced at an exhaust shop or do it yourself if you are experienced with exhaust work. You can get a new replacement cat for your Jeep for $100-$250. These will either be weld in, clamp in or maybe bolt in.  
P.s. I am not Dr. Vern (but I am happy to take credit for his writing!)

yah were im at people at people hollow out the catayltic coverter or run open headers and they will issue citations and tickets for noise pollution, some of them have the most hacked together trucks and jeeps like they have tires that stick out way past the fenders no mudflaps, drinking and driving, thinking the triple double roll bars that are thin chrome tubing will protect them from roll over, front lift blocks etc  

Appreciate the info. The cat is rattling and have been told that it should be replaced. Was curious if any increased performance would be gained after removal.