Yep, some gamma magic, the following algo reproduces the unreal result:

pow(pow(image1, 2.2)+pow(image2, 2.2), 0.45);

I made sure that the post process chain is empty. Also tried 2.2 power, then the multiply, a closer but still very different.
Photoshop's Linear dodge is exactly the same as the Unity result so you're off the hook. ;) Still, any thoughts or ideas are welcome.

Cheers! :)

Indeed it helps. Weirdly I can't link one custom code to another for some reason which limits its use.

Puh-leez! :)

I wanted the bumps. :) The wiki is great!

A dirty fix is this:
Clamp0-1 <- Add(0.0001) <- Sin <- Time

Hm I've just noticed this error in the console:

D3D shader assembly failed with: (11): error X6061: _sat not permitted with SINCOS instruction.

Oh yes sorry: the quad preview mesh, 50% gray background, dx9.

After turning off automatic preview generation everything sped up considerably. The last case in the table above now loads in ~25 seconds.