The car's mileage would be helpful. Since you have done all the other stuff in the recent past, and the car is getting 24mpg, it can't be that bad.
Does this one have the kickdown switch button under the pedal when you floor it? Besides that switch, the tranny's internal kickdown shift is based on line pressure in the valve body and the lambda between that and the data from the speed sensors. You could get check/replace the speed sensor in the tranny when you drop the pan for the filter service. Finally, the tranny control may very well have programming to disallow kick down above a certain speed, but from your description of the car's prior performance, probably not the case here.