Did some tests and realized that one frame in Full HD would take 3 hours and 45 minutes to render on my quite new computer, which isn't the slowest one.
So I had to change some things:
-I disabled motion blur. I'll add it in PW.
-Reduced samples for soft shadows. Tried it without softness. Didn't look good.
-The smallest size of the tarmac's power fractal for the displacement was insanely small. Don't know why I hadn't realized that. Increased it by a factor of 50!
-I deleted each reflective shader and used the specular reflection of the default shader instead. For some reason there's some real reflection of the environment as well, but renders much faster.
-Deleted each glass (water) shader as well since you won't see it anyway. Just made the parts invisible that represent glass.
-Unchecked Detail blending. I know this should be on, but I'll do some tests without and check if it's working.
Rendertime's now 45 minutes per frame! Not bad. Thinking about rendering the whole thing on my machine. I guess the file would be very very large if I'd gather it for sending it over to Pixel Plow especially because of the birds object sequence. Still undecided, but we'll see.