Reading my initial post, the problem statement is not accurate.
My problem is actually the insane amount of texutre-sampling ops I'm doing, which is implied from the number of render-passes I have.
The obvious solution is to reduce the size of the kernels, which I tried, but in some cases it creats noticable visual artifacts. Another solution would be to use less TXS-heavy techniques - I've tried a couple of different techniques, I'm pretty happy with I do right now.
That leaves render-pass reduction/merging.
No reason you can't use the bloom texture as input for average luminance
Your'e talking about the bright-pass texture, correct? It contains a lot of blacks, so can't use it as-is for luminance. I do like the general idea, though. I currently don't use the alpha channel of the bright-pass map, can store the luminance value there.
Tonemapping can trivially be slapped on the end of the pass immediately preceding it, as can film grain.
Tried that, very minor performance improvment.
You can merge depth of field and motion blur calculations somewhat by using a skewed disk sampling pattern
I was thinking of doing something similar with DOF and subsurface-scattering. It does look like a lot of work (both coding and doing math). I wonder if it's worth it, performance-wise.
I've reached my performance goal (which is good), but with very little performance to spare(which is OK for now).
Still, I'm very interested to hear from others on their post-processing pipeline approach/optimizations/design.