To my knowledge, the last major upgrade to the Harrier was an extended composite wing designed by McDonnell Douglas which gave extended range and carrying capacity, but at reduced speed and turn rate. BAe designed a GR5K "tin wing" Harrier which maintained the turn rate.
This "tin wing" could have been retrofitted to the existing Harrier (and Sea Harrier) fleet giving a faster, more manoeuvrable aircraft with more lift capacity than the AV-8B, but the US Government would only buy their own AV-8B design, and would only purchase if the RAF also purchased it.
As a compromise BAe designed metal wing root leading edge extensions which gave improved turn rate. These were eventually fitted to both the RAF and US Marine Corps versions of the AV-8B. The wing root leading edge extensions are metal because McDonnells refused to export the composite materials technology to BAe.
Note that the eventual AV-8B/GR5/GR7 is actually slower than the GR3/FRS1