Finding matched furnace with Trane XV18 Heatpump

Hi, I am looking for second opinion. A local hvac company quote me for Trane 4ton XV18 heat pump and pair with S9V2C080 (80k btu, 96%); need 11.7+ EER2 and 17+seer2 for state incentive I asked if it can be pair with S9V2B080-VS furnace (80k btu, 97%) instead because the later would qualify for federal tax credit, and was told that the later pair would reduce EER2 from 11.7 to 11.2. Is the more efficient furnace actually reduce EER2, can I have second opinion? I was told pairing the XV18 with S9V2C100U4VSA (100k btu, 97%) would meet the EER2 11.7 but that is oversizing. Second question, what is the cons of oversizing from 80k to 100k btu gas furnace.

Thanks