So you used diagnostic photon to see the density?
When you say a spotlight of 179 degrees do you mean almost a full hemisphere? That will be a lot of photons required.
How many photons for the light?
What are the materials on the bounce surfaces?
Do they have appropriate photon shaders?
Does the HDRI skydome emit photons?
Are you using final gathering (FG)?
How many bounces on average do you want?
Consider using FG only with secondary diffuse,
if less than 3 and you are in Maya 6.5
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