What evidence do you have to back it up?
I have my own theory;
When it starts to rain it is cloudy. When it is cloudy, there is no bright sunlight, and that means
they could tone down the lighting/reflection effects during rain to make up for the extra processing power needed for the rain effects, without reducing the visual quality of the game.
Simples.
Nope.
It's not the quality of the lighting effects that's the problem. It's that they exist. You can turn the reflective properties of a material down 50%, it still uses the same power.
Think of it as typing 1111 + 1111 into a calculator. Uses the same amount of memory as calculating 2222 + 2222.
That said, there are a few things to think about for rain.
Firstly, the track. It's gotta be darker, a darker flat texture (which would be used for the tarmac) needs a bit more contrast, which drives up texture size.
Then, and probably most importantly, water pools and reflects! Reflection = huge performance hit. And these won't just reflect the world, they need to reflect the cars moving in it, so double that performance hit.
Add in some dry and drying lines, another hit.
Now to the cars.
For the strange uncivilized people who use chase cam, you'll need water flowing over the body of the car. This is a nightmare - remember those scratches? You've got to animate them to move over the texture of the body of the car.
Then there's the spray coming off the car, it's wheels, spoilers (remember those flick up a lot of spray). Nightmary.
For us legendary gentlemen who use driver cam, we've got wipers. Easy to animate the blades, pain to animate them WIPING OFF WATER FROM THE WINDSHIELD. And the way that happens will be different at different speeds, because the water won't fly up and over the top if you're standing still, will it?
And that spray has to appear in the rear view mirror (can't forget those).
That said, there are some tricks and gains to be made.
You won't be able to see as far, so you can drop the LODs for anything and everything by about ~35%. Save a couple of hundred thousand polys, a few meg of textures.
Shadows on the world won't be as noticable - those small things that cast visible but faint shadows in full light don't need them anymore. And you don't need anywhere near as much quality in the rest of the world because you're not going to see it.
Actually, those are the only two 'good' things to come from racing in the rain. P.D., good lock and god speed.
hahahaha, Polyphony, speed, etc
They use normal maps on the interior, though, and I wouldn't be surprised if certain external elements (tire tread for example) use normal maps.
Pretty sure everything on screen for more than 10 seconds in every game released this generation has a normal map. Poly budgets aren't THAT high...yet.
