You have now found out what has always sucked about making complex tracks ...
It is Always a compromise when building any track ...
It is not just the number of polygons for a given area ... its textures as well, How they get rendered, the size of the texture, does the texturing have Bump and specular rendering as well ...
All these these things are fair game as far as the CPU/GPU is concerned ...
So you must always be aware of "How much stuff" you place in any drivable area ...
For instance ... the number one FPS killer on most tracks (Crank up your system to get the maximum graphic display to witness for yourself) is the Pit road and start/finish area ...
Why? this are always seems to place where most love to go overboard in detail.
Don't get me wrong, eye-candy is great, but you don race on eye-candy ... you race on the racing surface and the terrain that surrounds it. You must always compromise to achieve the best FPS without destroying the "RACE" ... Atmosphere vs. Race-ability ... How may cars are you going to have on track visible? Is it a Night race (A REAL FPS KILLER), Is it going to be a tightly joined pack of cars in a small circuit, or will the cars be spread out ofter a lap or two?
All this is part of the compromises ... you have to always be willing to give up something to have something else ...
For me, ITS the EYE CANDY ...
RACING experience is #1, What I drive on and and the terrain the course is laid out on always wins over "How main grandstands with people taking pictures and seeing flash bulbs going off", or "driving down pit road and have a fully decked out tool box in the garage, and being able to count the number of tools in each box".
It is great for effect but does not do squat for when you are racing, other than drain off FPS ...
Just my humble opinion ... So cheat where you can get away with it in terms of creating a forest .... when you have lots of objects to place and when close to the racing area