The unit is not apt to be operating at its rated SEER and Btu/hr level, therefore, the SEER is rather irrelvant.
When a typical HVAC contractor quotes the efficiency of the Air Conditioning equipment SEER & Btu/hr and leads you to believe the new equipment will automatically deliver that efficiency & Btu/hr, think again.
Typically, installed equipment only operates at 55% to 70% of rated capacity.
Its important to understand that "equipment ratings are only the potential efficiency of that component of the system under perfect conditions." Over half of the systems efficiency depends on the duct system and the field-installation.