I think they shouldn't impact performance in any meaningful way, especially the weather effects. Night racing though is an entirely different matter.
In my opinion, even the most powerful PC in the world couldn't run a game that tries to match the shadows cars cast in real life during the night. Every object occluding the light from the cars cast different shadows, and they are always moving. I wonder if we will ever see a racing game casting shadows correctly in a night race. Perhaps not in my lifetime, who knows, they seem to impact performance so hard.
I think a game console that uses a PowerVR based MCM configuration could do all the shadows but it would be hard shadows? I remember back in the day when PowerVR used Infinite Planes rendering which allowed objects to cast shadows. Maybe Kristof knows if it could do it or not with today's PowerVR tech?