Ok, I figured it out. Long story short: I had been meddling with settings too much, both in-game and in the Nvidia control panel, which made things run like crap.
That being said, I can proudly say I'm running Sikkmod with all bells and whistles enabled and properly tuned. Took me a couple of days but I'm finally there. Only thing I can't do is AA + AF combo, that's too much for my setup. Well I guess that applies to almost every current setup, right ?
Gonna put some footage on Youtube soon.
Now that I'm here, I would like to reply to your opening statement. You say that SSAO and soft shadows take a big performance hit with little reward. I strongly disagree. In a game such as DooM³ , which in fact is a complete showcase for shadows and dynamic lighting, soft shadows and SSAO make an immense improvement in visual quality ! It's true they really "bite" your gfx cards in the *ss, but the improvements are sooo good it's hard to ignore them IF you have the power to enable them.
Soft shadows is my favorite enhancement. The overal experience imroves dramatically as shadows are cast 10x more realistic than without. As SSAO is concerned; also here the general look and feel of the game is improved, yet not that big as soft shadows does, but still it creates a much more vivid and realistic scenery.
Like I said I've been playing around with settings for days now, probably more than a week of constant tweaking, settings changing, comparing images, and I have finally reached proper settings for almost all sikkmod options. I'm just one of those guys who enjoys tuning and tweaking it to the very last bit just as much as playing it, haha
The lighting with HDR and bloom, the reflection with SSIL, the soft shadows with SSAO (HBAO High), and all other settings; DooM³ truly is reborn !!
Only things I'm not using, are the ones that I'm not convinced of to create a better experience: depth of field, motion blur and color grading.