Manifolds and cats

I have a 2010 SE that has issues at about 45 mph going up hills on the highway. Took it to dealer to have it checked and said no codes but had a random misfire on left bank with 3 of the cylinders. Said that cat was at threshold 2?? Converter was starting to go out. Truck has never thrown a SES. I had a K&N oiled filter and removed it went back to paper. Cleaned the MAF but still the same. So ordered a new MAF yesterday. Going to try that first. If the cats are going out should I replace with new or gut these and move the O2 sensors out with defoulers? Is this going to hurt much or anything if I do since it has the other 2 cats still on there? I would do headers but not looking for gain anything since Mrs drives it on occasion. If that doesn't do the trick then it may be the fuel pump???