I think people arguing that Forza doesn't push polys in the interiors haven't seen those :
While they don't show all this in gameplay (no looking back inside the cockpit like in GT), the assets are all there for almost 700 cars that we know of (FM4 and FH combined car list) and the full list of cars they have modelled is probably way longer.
I've been saying for years that Forza's models are incredibly detailed and that they just need the right lighting and shaders to fully shine. Horizon's lighting engine and garage showcase this even more. All models from FM3 onward are just top notch in term of modelling, it's just the leftovers from FM and some from FM2 that are poor (dat 22b, dat F40...).
I would love to see some of Forza's model in GT5's lighting engine, it would create some amazing results I think...