Steps to reproduce :

1) Download attached project "TestUI.zip" and open in Unity 5.4.x .
2) Open Profiler and press Play.
Check in Rendering Profiler DrawCalls:2(see attached image "alpha255.png".
3) Stop the game.
4) In Hierarchy tab, expand Canvas, select all images inside and set their Alpha channel to 0 from Inspector tab.
5) Press play and check Profiler.
Check in Rendering Profiler DrawCalls:2(see attached image "alpha0.png"

Note: Difference between 5.3.x and 5.4.x is that 5.3.x version makes DrawCalls:3 and 5.4.x makes DrawCalls:2, but when alpha channel is set to 0
makes no changes in DrawCalls.