[mbridges]

The AC system produced supply temps. At 57-60 degrees on the first floor
With a return temp. of 71 degrees. The second floor supply temp. is at 59 degrees with a return temp. of 72 degrees. I know there should be at least a 15 degree temp. difference from the supply to return. There was also an
Air leak at the HVAC unit in the attic at the unused condensation drains but
A cap was placed where the leak was so could that take care of the degree
Difference in the supply and return?

[Flickit]

The AC system produced supply temps. at 57-60 degrees on the first floor
with a return temp. of 71 degrees. The second floor supply temp. is at 59 degrees with a return temp. of 72 degrees. I know there should be at least a 15 degree temp. difference from the supply to return. There was also an
air leak at the HVAC unit in the attic at the unused condensation drains but
a cap was placed where the leak was so could that take care of the degree
difference in the supply and return?

... degree temp diff? Anything lower than this would be acceptable, especially on a second floor. This number is probably typical and is considered a ballpark to shoot for.

[mbridges]

It was quoted by a home inspector. Could fixing the air leak help with the
temp. difference?

[Flickit]

It was quoted by a home inspector. Could fixing the air leak help with the
temp. difference?

... could go either way in some cases but in general closing any air leaks off when a closed system is in place (furnace, A/C, etc.) will help improve the efficiency and reduce the temp diff. A swamp cooler, on the other hand, needs air leaks else it will overwork.