update: I guess I wasn't measuring the temp correctly - as an incoming air temp i considered the thermostat reading. When reading the air temp incoming into the unit and it was actually several degrees higher. with that, delta temp is around 17deg which is normal. although, due to the AC working for both zones (upper floor has no buffer from the ambient besides the roof and gets super hot) it's not enough to cool it down deeply. will be installing split system just for the second floor - that way temp upstairs is controlled and no hot air being sucked into the return. Mr COOL - here it comes.