Look through the documents on this page:
http://www.spitfireperformance.com/spitfire-V.htmlAs to why the change was needed, well, we didn't have anything for 1941 or the first half of 1942. The Spitfire Mk IX in AH has an earlier service date than the +16lbs boost Spitfire Mk Vc had, so essentially we went from the Spitfire Mk I to the Spitfire Mk IX.
We used to have:
1940: Spitfire Mk I
1941:
1942: Spitfire Mk IX
1943: Spitfire Mk V
1944: Spitfire Mk XIV
Now we have:
1940: Spitfire Mk Ia
1941: Spitfire Mk Vb
1942: Spitfire Mk IX
1943: Spitfire Mk VIII
1944: Spitfire Mk XIV, Spitfire Mk XVI
The overboosted Spitfire Mk V is not representative of a 1943 front line Spitfire.