Well the merlin had pretty much reached it's development potential. The Mk XI may have the last production Spit with the merlin, not sure, have to do some more research.
In order to increase power more displacement was needed. The merlins displacement had reached it's max as a production engine. So when Rolls Royce enlarged the engine they decided to call it the griffon. A MkIV was the first to be test flown with a griffon. The griffon also turns in the opposite direction from the merlin.
If you'd like to learn more you can find two or three books about WWII engines or you can read Spitfire the History by Eric B. Morgan and Edward Shacklady. It goes into great detail about all the variants. Especially engine and airframe development. Great book if your a Spitfire person. [&:]