When the detail settings are near identical the performance is also near identical. Don't kid yourself. While I agree not every ultra setting is worth going for... I do get a much better sense of immersion from this:
The thing is, the little things DO matter. And they are mostly in additional shadows and lighting, plus LOD on more distant objects. Look at the vegetation further away and its detail, for example. All that LOD is going to shift and change as you move around the world. Not nice.
What Ultra offers, overall, is a far more stable, natural picture. The lower you go in settings -
even to very high - the more you sacrifice in those qualities.
I don't know man, it looks like there is far more than an AO pass going on there. The contrast is heavily increased, its almost as if someone went crazy with SweetFX and overdid it all.
While I agree the shadows 'pop' much more, you can also see that many colors are simply crushed into pure whites and blacks. HBAO+ has more variations of shadow depth. In the first picture you can see the note on the box to the right being glowy white (wtf?) while on HBAO+ you can see the lines of text on it and the box isn't crushed into blacks. On the second picture you can see that HBAO+ offers more shades of darkness in the shadows too.
That aligns with my example above - look at the vegetation in the shadows under the trees (middle of pic). The HBAO+ Ultra screenshot allows you to still see the individual leaves clearly. The High (SSBC) screenshot completely darkens that area, and it barely interacts with the god rays.