Why do the old A320s have a lower tailwind limitation compared to the newer enhanced ones?
Short answer: the increase from 10 to 15 knots requires extra certification / flight testing, which is an extra item the airline will pay for.



Over at pprune:


  Boeing offers an increased tailwind limit of up to 15 knots for a fee.


I suspected that but had no proof, so that got the ball rolling.

From page 35 of an Airbus presentation presented to the civil aviation authority of PRC confirmed that being an option:


  Other configurations outside China include options as:
  
  
  Water/waste freezing protection for extended flight conditions
  15 knots tailwind certification at landing
  15 knots tailwind certification at take off
  


And finally from the FAA's Flight Test Guide For Certification Of Transport Category Airplanes, there is a section for certifying takeoff and landing in tailwind greater than 10 knots.

It requires the demonstration of controllability, engine performance, autoflight systems, landing gear vibration, terrain awareness warning systems to not be affected by the higher sink-rank, etc.

Note that for a takeoff in tailwind >10 knots, even the application of takeoff thrust is different from when &lt;10 knots. The advisory circular also calls for a disclaimer of sorts to be added in the AFM:


  The AFM should contain a statement that the limitation for tailwinds greater than 10 knots reflects the capability of the airplane as evaluated in terms of airworthiness but does not constitute approval for operation in tailwinds exceeding 10 knots.


Of course if the airline would also pay even more for the brake cooling fans option, they can reduce the longer cool down period after a tailwind landing, for example.



Some performance enhancements require extra certification and testing, but not necessarily extra equipment.

An airline can request their fleet to be certified for a lighter or higher MTOW depending on their operation. A light MTOW on the certification is cheaper to obtain, and it will allow the airline to save on ramp fees, route charges, etc. But on the other hand they can't carry as much payload/fuel. Other high MTOW options may require extra strengthening built into the airframe and more powerful engines.