Is your o2 sensor in the correct bung? It should be in the farthest back bung, ie closest to the muffler. CEL could be in related. Have the code pulled and make sure it isn`t something worse.
Good inverstment is a OBD scanner.
I dropped .3 in the 1/4 mile and picked up about 2 mph with just the MKIII addition, on an otherwiae stock car.
Bill