IMO a 6-8 degree spilt is to low. Can not see it from here. With temps like yours my unit would put out 50 F air. At 81 F inside it will reach 60 F out put air temp. But that is not what your asking.
Change the filters, check the fan for speed setting, probably on medium with any luck Check output of ducts with more that your hand. the obvious, heat rises I will bet you do not have a return upstairs.
Sounds to me like there is a problem with the volume of air to the upstairs.
As a QUICK FIX not to be done for anything other than testing take the filter out and see if there is any difference. Some units are VERY sensitive about static air pressures, installing for example a hepa air filter will lower the air volume to a point where the unit has unsatisfactory performance.
All are WAG's