The Spitfire Mk IX was the "stopgap" produced to counter the Fw 190A by fitting a Merlin 61 engine into the body of a Mk V; because the proper Merlin 61 plane - the Mk VIII, wasn't ready. The Spitfire F.Mk IX in AH currently performs like the more powerful HF.Mk IX in top speed only, ie: it's about 10mph too fast. It needs to be fixed.
The first Griffon(III or IV) engined Spitfire, the Mk XII was also produced to counter the Fw 190A, mainly because they were making low-level jabo attacks over British territory. They were operational in February 1943.
The Spitfire Mk XIV was the next Griffon(65) engined Spitfire. While deliveries to units started in January, they did not become operational until June 1944.
Pongo - if you want a better Merlin engined Spitfire, to avoid confusion it should be either a Mk VIII(F,LF or HF) or Mk XVI(LF). The main difference is that they can climb faster since they have about 100hp more power.