You highlighted the one thing I already said I agreed with. That area of Spa is too grimy but the rest is accurate.
Also looking closer at the Spa image I realise now that GT has an inaccuracy on the barriers I mentioned. The mesh fences are a lot shorter than real life/Forza and look to be greenish instead of grey metal. Whether this is a seasonal or age thing I don't know, but it doesn't match the other two.
I still don't see the reason behind "Forza looks rushed in comparison" when the tracks are essentially spot-on, especially in terms of environmental details.
Is not only the fence, many things inside the red square looks wrong when compared to the real thing.
The small green GT fence is also present in Forza behind the front tall fence. Probaly the GT track was modeled in a prior stage when the taller fence was not present in the real track. Or maybe they have no more ram available to model a double fence on that track...
Look at the bad scaling of the Coca Cola logo at the tower roof, at the lack of the red paint discoloration, at the inaccurate antena, at the lazy texturing work at the building side wall panels, etc... there are many details that scream "rushed" or lack of love. Forza It does have more elements modeled but that is something expected running in a better hardware, same as with high resolution textures, but the extra care that PD put on their tracks is not there in Forza.