From what information I have, the MkIX was still very rare in 1943, and IXLF beings even rarer.
Qoute from Alan Deere, Biggin Hill, Wing Commander Flying
Unlike the Spitfire IXA, with which all other Spitfire IX wings in the Group were equipped, the IXB's supercharger came in at a lower altitude and the aircraft attained its best performance at 21,000 ft,
According to that qoute, there were only in pennypocket numbers, maybe 2-3 Squadrons equipping with it in 1943, the '1942' Merlin 61 version being the dominant type of MkIXs.
Mike Williams claims this to be dated March 1943, but wheter the qoute is true or 'doctored' I don't know, what I know that Alfred Price also qoutes Deere, but that qoute is different than Mike's version of it.
As for the July 1943 strenght of Fighter command, Spitfire strenght was dominated by MKVs, about 4 times as numerous as MkIX (all engine versions). Note that the MkVI is just a MkV with the same engine, and some provisions for high altitude work.

In short the MkIX did not became the 'main battle line' until 1944, by D-Day there were some 34 or so Squadrons operating on MkIXs, and a few still on SpitVs and some even on Hurris.
During 1943 it was the MkV that held the battle line, facing Bf 109G2, G-4 and G-6, and FW 190 A4 and A-5.