No real secrets tbh. Basic layout from google earth. Height/elevation data from Nasa SRTM dtabase which can be used to generate a displacement map for the terrain mesh. As for fine tuning the layout and creating environment - well that's countless hours watching videos and gathering screenshots.
Collecting accurate reference material is the longest part of the process by far. Especially with a track like Barbagallo because it's not that well known outside V8 circles and it recently underwent a major redevelopment which means up to date reference material was very thin on the ground.
No shortcuts, just shedloads of donkeywork, cross referencing and testing until it looks and feels right.
Modelling done in 3DSMax (never been a fan of max but it's really the only option for getting seamless workflow in/out of sims). Most of the textures are hand drawn in photoshop, some i already had from previous projects and some (such as logos) are from the internet. Often these are too low res so i'll convert them into vector images so i can scale them up.
You're right about the amount of effort in doing something like this - especially when 90% of everything is designed from scratch. From conception to release I reckon l'll be close to 1000hrs
As you know, Kunos's engine is impressive in so many ways but it lacks support for any real environment immersion.
I'd like the ability to turn objects on and off between sessions, support for multiple layouts without having to duplicate assets, more parameters for audio objects (to actually make them useful), much better support for collidable objects (weight, inertia, density), more control over particle effects, definable tyre wear per surface, animation support (preferably with the ability to be triggered), separate pit and garage positions, per track lighting/clouds/fog and flag rules with the ability to alter objects/lights depending the level of caution.
It might seem like a lot but the gmotor sims have done most of this for 10 years. Hopefully we'll get there in time.