There is a heck of a lot of myths in this thread; no such engine as a "modified XX" was ever fitted into the Spitfire. The XX had a single-stage, two-speed supercharger; up to, and including, the VI, Merlin superchargers were single-stage, single-speed, while, from the VII onwards, they were two-stage, two-speed. The III was abandoned largely because the Merlin XX was needed for the Hurricane II, and the Merlin 45/46 (basically a modified III) proved entirely capable of doing the job of coping with the Me109F.
Another advantage of the 45 over the XX was the ability to get it into the same space as the I/II/III series, so there was no need to modify the area in front of the engine bulkhead. If you look at the photos, you'll see that the Mk.III bulkhead angles forward, at the top, like on the Spitfire XIV; this led to the III fuselage being 4" (not 30" - where the heck did that come from) longer than that of the I/II/V/VI. This also led to the u/c being raked forward 2", which led to the "universal" C wing, which caused problems leading to a curved oleo cover, instead of the flat type seen on the early Marks.
N3297 eventually became the prototype IX.