Dude this is a common prob in MR basically because theres not enough literature on it. But...

The min and max radius settings are pretty much 10% (roughly) of your scene scale. Best way to work it is to put in a plane and scale that to your scene size, then take the settings from that. So if its scaled to 100 x and z, your min radius would be 1 and max 10.

These are pretty much the right numbers. The max radius is done first out of them then this way you calculate the min radius (10% of max) otherwise without these adjustments Maya is automatically figuring it out hence the dirty appearance. This will result in a cleaner render. You can lower the figures but they have to have the 10% difference of the two, you can then increase your final gather rays. This SHOULD work, honestly.

Cheers
Jay