Might be something to take in for. These have a small catalytic converter inside that runs hot anyway..but if there is something extra introduced at the right quantity, like extra fuel..as in in a sticking or leaking injector, or even oil, they can run much hotter then normal...even hot enough to close-up and cause additional restriction..which would cause loss of power. Of course there are a number of other things that can cause power loss as well...many we don't want to think about...and some that would cause oil to be introduced into the system. So..maybe you could take the muffler off just long enough to see if it runs normal again and no oil smoke. It's good to know if one of the O2 sensors gets contaminated enough to cause an error code, that the code will tell you which one...and this which cylinder has the problem. Of course it could also be extra air getting into the exhaust system as in some spring came loose and the donut has lost it's seal. Check those too but sounds like that damage is already done.