Stalker Clear Sky engine dissapointing

I've just started to finally play Stalker Clear Sky, as I have been waiting for a decent patch to fix the numerous bugs and crashes.

So I go into options, turn everything on max (dx10), start a game and notice the very low FPS (relatively smooth in some areas, very laggy in others).

I go into the options menu once more and turn off options one by one to see what the problem is. All of a sudden I turn of Sun Shadows and the fps goes 40+.

I can't understand why this engine is so un-optimised. Crysis Warhead would work at max with very nice fps and temps never got past 75C.

But with Clear Sky's X-RAY engine, sun shadows seem to make everything slow and laggy and temps go well beyond 81C in this game.

Just wondering if you know any mods / patches / tweaks to make this game work well. A lot of other people are complaining.

Turning off Sun Shadows doubles the fps, but then the game would look awful.

The Swamps have 6 million polygons, which is 3 times the number of polys in Warhead.

The other areas have 2-3 million, and the game runs smoother there.