you forgot about dynamic shadows and more elements of lighting combined into total image...
Actually I would in place of Scawen use simple answer: "yes, FPS will go down a lot" ... if I'm wrong, people will be happy that it is optimized well. If I'm right, they will hear it soon...
Can't lose. (except there will be some moaning of people who didn't buy new computer in last 5 years, but there's no alternative any way, either the LFS is still in development and progressing toward being best simulator possible (and then it needs some HW power and it will surely not scale down over years, actually a high quality simulation will very likely eventually need higher frequency physics which will be another big step in HW consumption), or it's a toy game for low-spec PCs and then Scawen can quit his effort...