Summary:
1. For PNG rendering, Depth map and Distance map bitmaps are only 8bit, while RGB and all other channels are 16 bit.
2. RGB images are 72DPI, Depth Images are 96DPI

Description:
When rendering images with a depth map using PNG format,
the RGB image is rendered using 16bits per channel PNG, while the depth map only uses 8 bits per channel (giving only a maximum 255 levels of gray for depth compositing as the depth is duplicated on all channels.)
An 8bit Depth map gives rough edges and lots of artifacts during compositing.

The Depth map should also use 16 bits per channel so that there are at least 65535 levels of depth in the depth map to allow smooth compositing.
The DPI of all output maps should be the same (either 72 or 96) to avoid costly stretching during compositing.

To recreate:
In iRay Render dialog, select all output channels: RGB, Depth, Alpha, Emission, etc
Render a sphere using PNG format
Make sure "Transparent background" is deselected.
Observe the RGB Image is output as 48BPP with 72DPI.
Observe all other images are output as 48BPP, with the exception of the Depth and Distance images.
(In Photoshop, Open the Image->Mode menu and confirm each channel is 16bit)
Observe the Depth and Distance images are output using only 32BPP with 96DPI.
(In Photoshop, Open the Image->Mode menu and confirm each channel is 8bit)

Expectation:
Depth and Distance maps should be rendered with at least 16bits per channel
DPI for all output images should be the same

Attached examples:
8bit Depth images and 16bit RGB Images from the same render