2002 Chevy Malibu

I have a 2002 Chevrolet Malibu with 112,000 miles. The vehicle has hesitation on acceleration when put under a load such as going up a hill. I have just replaced the plugs, air filter, fuel filter and egr valve due to it finally throwing a code. The code revealed bad egr which did not solve the problem. I removed a o2 sensor in front of the catalytic converter and at idle it showed right at.05 pounds and at 2500 rpm 2 1/2 pounds. I was told that is a little high at the 2500 rpm reading. Do you think the catalytic converter needs to be replaced and would you purchase the part from GM or the Walker direct fit from the local parts store which is half the price. Also the vehicle gas mileage has dropped almost six mpg over the past six to nine months. Thanks for any help.