My '06 Impala A/C works well, or seems to, until the outside temp gets into the mid 80’s. Rented cars have been known to “freeze you out of the car” but the dealer has seen it twice and supposedly, everything is Ok. Warranty runs out in 2K and now it is fall and I won’t use the A/C until next summer!
Perhaps the flap that directs air through the AC evaporator is not moving far enough, and some of the incoming air is being directed around, instead of through, the evaporator, which would mean some of the air is not being cooled.
I’m assuming the AC system is full of refrigerant, there are no leaks, and the pressures within the system are correct, as per the dealer.
In the Factory Service Manual there is a chart that specifies what outlet temp you should have within a certain set of parameters, the chart is totally objective, when I would be dispatched a car with a “poor AC performance” concern I would staple a copy of this chart to the back of the Repair Order to show the customer where their system sits, unless the car really was performing poor them I would fix it