Details

Since _rt_ResolvedFullFrameDepth is IMAGE_FORMAT_RGBA8888 and only using red channel, it has very little precision. Custom shaders were allowed recently, but they still suffer from imprecision because of this limitation. Upgrading the depth buffer will allow for better screenspace shader effects, such as SSAO. For now, it's not possible because of a crappy 8 bit depth buffer.
This removes large artifacts from using depth buffer in custom shaders (Normal reconstruction is an example of that)
But it might not work with default DOF with gmod shaders.

Proposed solution

Make a separate _rt_ResolvedFullFrameDepth32 rendertarget with the IMAGE_FORMAT_R32F image format, which will be copied to the good old rt_ResolvedFullFrameDepth rendertarget with IMAGE_FORMAT_RGBA8888 image format. This preserves backwards compatibility, whilst improving precision.