(2017.4) Suggestions for UI Camera Replacement Shader?

We're using a 2 camera set up in our game - one to render the environment inc. post process stack, and another for worldspace UI, with it's own post-process stack + some additional affects. (This is done so that the post process settings are not applied to the UI, and the UI can layer on some additional effects onto the whole mix.

I've noticed however that in the GPU Trace we're seeing RenderOpaqueGeometry -> RenderDeferred.GBuffer, RenderDeferred.Reflections, RenderDeferred.ReflectionsToEmissive being called and using up nearly 1.5ms before any UI Canvas is drawn, or Post applied. Is this a necessary requirement, or are there ways to bypass these somehow using a replacement shader (or similar).

Any help much appreciated!

 

Set your UI camera to use Rendering Path > Forward