Well, it does make some sense to do so. You are right that iRacing, AC do not have fully dynamic lighting. But PCars2 does. And I would say, even on a relatively potato PC like mine, I am seeing less variation in running between normal weather, night, rain and night+rain. Yes there are significant fps drops, but not to the same extent as rF2. And at those, it's still just about playable with about the same graphical quality. But SMS have had a lot more time to optimize their engine than S397 have (two major releases) and they also have to contend with multiple console architectures as well. They both use FMOD and YEBIS.
iRacing just released Belle Isle. Where I can see 150-200fps alone/80fps with field on Spa for example (even with the latest improvements to shadows being cast on many more objects), I'm down to 90fps alone/sub 60fps with field on this track. There's a lot more going on with the shaders and such on it, its a lot more graphically intensive than older tracks. Which is why I am a bit concerned here for fps on Sebring, which I don't doubt both has many more polygons and also textures/shaders than older tracks do that already suffer.