Suggest override to 5th gear, and run the speed that keeps you between 1800-1900 rpm. Do not use CC. If you want to use D, we have TCM custom tuning that will prevent 6th gear lugging, and 5-6-5 hunting while in CC.
These parameters should help the efficiency at higher altitudes particularly.
get me into hot water with State Emissions Control?
If you have an emission test to pass, you should leave it alone. If not, you have the (illegal) option of changing your exhaust and using a decent custom tune to circumvent the electronic consequences of the hardware change. Message me if you would like more information.