Hmm, I checked the docs, and it looks like even though you have the settings maxed, you MIGHT be getting limited.

Don't quote me on that, BUT, here is a snippet from what I've found:

Getting Started with Maya
> Rendering
> Lesson 5: Caustics

Adjusting the refraction levels

When the refraction levels are set too low, light rays do not refract through all of the surfaces enough to provide a realistic indication of a translucent surface when raytraced. This results in the object appearing opaque when it should appear translucent.

Increasing the Refractions setting allows the renderer to calculate what is seen through the surfaces of the glass by allowing the light rays to refract the correct number of times.

In the next step, you change the Refraction settings so that the bottle will appear translucent when rendered.
To increase the refraction levels

1. In the Render Settings window, select the mental ray tab and then open the Raytracing attributes.
2. In the Raytracing section set the following:
* Refractions: 6
* Max trace depth: 8

The Refractions setting sets the number of times a light ray can be refracted. When the setting is too low the ray does not penetrate through all of the surfaces. The Max Trace Depth setting limits the total Refraction and Reflection settings. By setting it to 8, you ensure that two reflection and six refraction calculations can occur.

Photons refract in a manner similar to light rays when rendering. Increasing the Refractions setting for the Caustic Photons ensures that the renderer can correctly calculate the caustics that should occur with the bottle. In this case, matching the Caustic Photon refraction settings with the Raytracing refractions is a good practice.
To increase the photon refraction settings

1. In the Render Settings window, select the mental ray tab and open the Caustics and Global Illumination section.
2. In the Photon Tracing section set the following:
* Max Photon Depth: 6
* Photon Refractions: 6

...ignore me if I'm totaly off base