A good wideband is well worth the money. That said, I don't know the algorithms for the computer to throw a CEL for lean running, nor how the ESS tune would affect that (if at all).
Perhaps see if you can plug up a generic OBD-II scanner and look at the PIDs for long-term fuel trim?
The only root causes I can think of are running too lean or charge temps too high. Did I miss you mention if you do or don't have an intercooler?