Screen-space reflections issues

[Zylann]

Tested on Godot 3.0 master
Windows 10 64 bits, nVidia GeForce 940M

I tested screen-space reflections on a mesh with a metalness of 0.5, but they don't look good. I don't know what to do to fix them, I dabbled a little in the options of WorldEnvironment but none of the settings gave a good-looking result.

Setting accel to zero and increasing the number of iterations to 256 seemed to improve a bit, but it still looks jagged and lag is noticeable:

Trying on a flat plane looks ok, but the sky is not reflected (solid blue color). Also there are some edge-clamp artefacts, very noticeable when the camera is moving (not sure if that is fixable given it's "screen space"... Maybe fallback on a static cubemap, or fade to sky? I read some interesting techniques here http://www.gamasutra.com/blogs/BartlomiejWronski/20140129/209609/The_future_of_screenspace_reflections.php, I guess ReflectionProbes are designed for this but I couldn't get a difference with them)

I was able to set a metalness of 2, which gave a more contrasted look. However it gets clamped to 1 so I guess it was not really allowed?
Also transparency seems to be incompatible with reflections, they disappear as soon as "transparency" is checked in the material.
Is there a way to make reflections "blurry"? I tried roughness but it has no effect.

Here is a video of the water plane: https://www.youtube.com/watch?v=NbkvuQWgnkg&feature=youtu.be

Another one of a demo I'm making, not focused on reflections but still noticeable artefacts https://www.youtube.com/watch?v=r-doDosEA3E

The project:
ScreenSpaceReflections.zip