The Merlin 61, 63 of the F and 70 of the HF are rated for higher altitudes in both supercharger gears (can hold rated power up to 30k feet in the case of the 70) and thus reach their top speeds at a higher altitude. Thanks to this their top speeds are actually higher than the LF's, though they are slower lower down. The Merlin 61 also has a lower propeller reduction gear ratio due to its origins as a bomber engine (.42:1 vs .477:1) thus the early Fs had some different characteristics in the climb.
"In theory all that would need doing to create a late production F.IX in DCS is adjusting the engine and associated flight model parameters of our current LF.IX, no 3D model changes or major work." - Not so. A lot of 3D work would have to be done. The late F IX with the Merlin 63 of 1943 vintage would lack the outer cannon stub, use the early short air intake, have the fuel cooling port in the port wing root, different undercarriage/wheels (and other features which I'll detail in a list below which also apply to the F IX). That's not to mention the work required in the cockpit to bring it to standard. For example, the F IX used the Mk V throttle quadrant and slow running cutout controls which are completely different to the LF.
Unfortunately if the current DCS LF IX were replicating a late 1942 conversion, it would not be an LF. They did not appear until the spring of 1943. The fact is, the current model is a bit of a mess. I'm sure the flight model is fine but the 3d model, inside and out (incl. many of the features in the cockpit) are incorrect. The "classic" LF Mk IX really came into being in November/December 1943 - by this time the Aero-Vee tropical intake was being introduced, the slim feed motor blister for the cannon was standard, as were the late elevators, beam approach aerial, late IFF aerial, booster pump, and other features. For our LF IX which seems to represent (or ought to) an early 1944 LF Mk IXc in ADGB and 2TAF service, the following features are inaccurate:
1. Wing fuel tank cap on the ammunition bay; this is a post-war "warbird feature"- this is however just on the texture mapping and can be removed fairly easily
2. Over-wing wheel well blister and reinforcing strake; late/postwar feature related to a change in angle of the wheel axle (from 8 degrees to 4) when operating off of tarmac; should be completely smooth surface above the wheel wells, in-sim axle angle is correct 8 degrees however
3. Scissor link undercarriage; very late/postwar modification - oddly enough, the model has the correct gear leg fairings and bay without the extra cutout for the links; should be straight spline type
4. 5-spoke wheels; common in 1942 and Mk V conversions but by 1943/44 a four-spoke wheel was the standard, sometimes with a hubcap
5. Recognition light behind the aerial mast; this was deleted in 1943 and should be absent by the time the Aero-Vee filter was introduced
6. Missing gun camera in starboard wing root - the original port wing installation was deleted in lieu of the fuel cooler in the F Mk IXs - the LF lacked this cooler, but when the gun camera later returned on the F (and LF) it was decided to standardise on a starboard wing installation - it is correct that early LF's lacked the camera but probably by late 1943 and definitely by the time of D-Day it should have returned*
7. Missing IFF aerial under the starboard wing
8. Missing drop tank deflection hooks
9. Both wobble AND booster pump; should be either or - early ones had the wobble pump, later the booster pump; ours should probably have singly the booster pump
10. Missing beam approach aerial and associated cockpit control (this is a variable feature as some had them, some didn't)
11. Missing remote contactor control in cockpit**
12. Headrest; this is a Mk V feature not present on Mk IXs
13. Erroneous radiator flap control; this was a feature of the PR Mk XI and is a common "warbird" modification in modern times, is however anachronistic and should be in fact the "Camera Master" switch; only the test button should be present*
14. Missing generator failure light
15. Missing the interconnected prop and throttle control; this was standard in the Mk IX by the time of the Normandy campaign and was one of the other features differentiating it and the F Mk XVI, which wasn't set up for it
16. Missing gun camera exposure control on port wall*
There are some geometric issues with the external and internal 3d models, be they missing features or outright incorrect:
17. Cannon fairings incorrect shape; in-sim they taper immediately from the root, when they should have a straight channel, then a second tapering piece which attaches to it (this tapered fairing is the same piece as used on the outboard cannon in the four-cannon configuration, the E wing, and even the single cannon on the Mk VB). I remember originally the model had the later cigar-shaped fairing; this would have actually been fine for 1944 but it was changed in 2017 and in doing so it's now inaccurate. Additionally, the plug for the outer cannon casting is completely flat, when it should be somewhat rounded
18. Cockpit windscreen piece currently replicates the version from the pressurised Mk VII, which had a greater number of bolts and seatings for them in the windscreen casting
19. The windscreen framework is missing bolts; these appear however to be included in the Spitfire cockpit update that's upcoming, judging by the Normandy 2 trailer
20. The outer wing panels appear to be a little too thick in section; it's hard to get a photo demonstrating this but it's something I've noticed since the module first came out in 2016
21. Front of tail fin and fairing very angular and awkward; should be a more graceful curve
22. Profile of the bottom of the rudder is slightly incorrect
23. Edge of gunsight mounting casting is round; should be cut square
*These only apply if the gun camera is mounted, which it should be for the Normandy time period
**This applies when the wobble pump isn't fitted, which should be correct for the Normandy time period
Here are a few photos demonstrating the state of F Mk IXs throughout late 1943 and throughout 1944 - the appropriate time period for the module.
Here's an early Castle Bromwich LF Mk IX (before MH434 was built).
I really don't think a hodgepodge of features is the way to go, I feel an early and late LF variant set would be more appropriate - and that's not even mentioning potential F or HF variants and their differences!