Quote - A few more statistics....

The more bounces you have, the less light you need coming from the plane emitters.  So the light is not significantly diminishing between bounces.

5 bounces, IC 20 takes roughly the same render time as 1 bounce IC 100.  So in general, you are probably better off minimising the raytrace bounces and maximising the IC.  I think Maxxx was using 5 bounces 50 IC, which sounds like a perfect starting point.  In theory, 4 bounces, 60 IC (which slightly increased ambient from the emitters) might give a slightly better result.  Or even 3 bounces 80 IC could be even better again - given all 3 options should render in roughly the same time.  From a bunch of test renders, it was hard to see much additional render quality from 4 raybounces verses 3.

Also, my comment above about turning off Indirect Light Quality is WRONG.  Increasing this slider removes the blotches.  Indirect Light Quality slider does not appear to increase rendertime exponentially.  At least 50 seems to be needed to convincing renders.

For my simple test scene, 3 raytrace bounces and IDL quality of 50 took about 5 mins.  Then with DOF on, it took 3hrs.  Then adding post filter 2 gaussian, the render took about 9hrs.

I was just about to update my post confirming your initial finding on ILQ too.  Just did a render to validate that finding, and it turned out totally not like I predicted.  It does make a difference.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.