Photometric Light Timing Test Iray (Failed?)

The purpose of this scene is to test

1.  Time vs. number of photometric lights
2.  Whether photometric lights still have an added effect with more than 7 in a scene (someone has anecdotally claimed they do not)

It contains free items from the Daz Studio Starter Essentials only (the Alchemy Chasm, the default HDR, and photometric lights generated within DS).  There are three groups you can hide or show to add or subtract their lights: A single photometric light, a group of 7 photometric lights, and a group of 50 photometric lights.

I'm rendering with 2x GTX 980, 2x GTX 740, and a Core i7 CPU.

My render results with the test scene are:

Single photometric light: 13 sec
7 photometric lights: 50 sec
50 photometric lights: 1 min 53 sec

Oddly enough, this appears to refute the claim that more photometrics are faster (something I've heard repeated as firm fact since Iray came out).  I can also conclusively refute the idea that 7+ photometrics have no effect.  The scene with 50 lights is noticeably brighter (which is why I've included it as the preview image for the test scene).

I redid the render with an identical group of 7 photometric lights set to a much lower light level (also included in the test scene) to control for whether higher or lower light level really has an effect.

7 photometric lights: 50 sec
7 DIM photometric lights: 13 sec

Confusingly, this appears to suggest the opposite of the doctrine that Iray prefers more light in the scene.  When I turned off the HDR (by setting it to Scene Only in Render Settings) it went to 7 sec, even faster!

A quick test of my own wherein I set the top of the roof to a separate material group and hid it made all scenes faster, but their ratios remained the same.

Next I will try again with the same lighting but more open geometry and see if that's making a difference.  Iray does not like partly or entirely enclosed geometry in a scene, maybe that's interfering somehow.