uGUI Image without transparency/ opaque render queue

I want to get the most performance out of uGUI as I can. Therefore I'm looking for a way to use fullscreen masked backgrounds (two side by side for sideway switching), rendered in the opaque queue, no need for transparency.
The UI shaders available are not explained anywhere I could find and if and which one of them would do the job and could be used with a mask would be interesting to know.

So how do I put uGUI Images to opaque render queue?