I would love to know the Carrier model numbers for these units.
Let's assume these are standard RTU's that are just running the OA dampers 100%......you're getting a 23 degree split and your maintaining a 79 degree space temp. What kinda of ambient temps are we talking?
Your manager said six years ago they were able to maintain mid 70's during the day and your at high 70's.......are you still at the same airflow and discharge temperatures. A slight change in either will cause a 3-5 degree change as you stated.