That's true, this mod will never be accurate for those reasons, and thinking about it, without those larger issues fixed, it's hard to pick up on subtler issues, like suspension settings (including front geometry, I only had one reference photo to go from, and it was taken from a less than ideal position).
Your point on tyre pressures is a good one too, what is the maximum that would ever be used in real life?
I've just done some more testing, I make better laptimes using R4 compound tyres, as I can run lower tyre pressures (50psi) without the tyres melting, which presumably more than makes up for harder compound. Increased rolling resistance takes a couple of mph from the end of the straights, but I'm faster the in bends, so all in all it acts like adding downforce.
I am finding the trailing arm suspension at the rear a pain, it's very difficult to prevent more load ending up on the outside of the tyre. Given that is the opposite end of the spectrum compared to a live axle, perhaps using double wishbone suspension at the rear would be a better compromise? It suspect it would make the beast a bit more tame to drive.
I am also considering testing dropping the CoG by a cm or so, and perhaps moving the static weight distribution back a little bit, but then the fuel tank forward a little bit, so the dynamic weight distribution stays at 51% rear regardless of fuel. Might make life a little easier for that front right tyre.