I don't think that the geometry rendered in the rearview mirror is all that memory intensive (or render intensive for that matter). It seems like it just comes down to how much is actually worth rendering in the RVM because it so small and takes up so little HUD real estate.
Keep in mind, the engine (and memory) keep track of where everything is while you are racing - the fact that it's rendered in a mirror mode is not taxing to the available computing power.
Now, if I am understanding you correctly lifestream, you are talking about "environmental mapping." That's the heavy load right there. That's intensive because the reflection that is (in effect) doubling the rendering load.
It was decent in the preview GT, and I also don't think it will get any better. There is some basic environmental mapping there, but it's faked - it still looks good, but it's not true env. mapping.