[hvacker]

Like BNE8EZ said about indoor design temps. A system could be installed that would freeze you out but you might not
want to pay the electric bill.
Some designers actually design for a 15degF over outside temps but I haven seen this for awhile and then only in track homes.
20 degF is more common. The effect of a larger drop can be too great to find comfortable. Then there's the cost of a larger system running when only at partial load. Running a 5 ton for a 3 ton load example.

The question is how has performance been historically. Was there a time where the system did better.