Screen flashed when changing RenderScale from >1.0 to 1.0 at runtime.

Anytime you change VRSettings.renderScale, it re-allocates the eye buffers, which can lead to dropped frames. But this ought to work without flashing a garbage image. It sounds like there may be a race condition between the re-allocation and the next buffer swap after it. We'll investigate. If you are varying the resolution on a frame-by-frame basis, you should use 5.4's VRSettings.renderViewportScale instead. It allocates only the largest necessary buffers, but uses a smaller viewport within them to save fill rate and bandwidth.

where can I find more info about the differences between  renderScale and renderViewportScale?

See https://docs.unity3d.com/540/Documentation/ScriptReference/VR.VRSettings-renderViewportScale.html and https://docs.unity3d.com/540/Documentation/ScriptReference/VR.VRSettings-renderScale.html.

what's the default value?

Yep, it's 1. That means the entire texture will be used for rendering. A renderViewportScale of 0.5 would use an 0.5x0.5 square inside the texture, resulting in a 75% pixel throughput savings. A renderScale of 0.5 would result in the same savings, but it would force a re-allocation of the eye buffers instead of using a smaller viewport within a large buffer.

 I could set renderScale to 2 (my max), then dynamically set renderViewportScale down to 0.5 in scene (or scenario) that I don't need the max level of sharpness.

Yes, renderScale * renderViewportScale would cancel out to 1 in that case, so the sharpness would be the same as 1, 1, but there would be a larger memory footprint.

On more question, for the later scenario (renderScale:2, renderViewportScale:0.5), do I also pay performance penalty twice? (e.g. perf implication from setting non-default renderScale + perf implication from setting  non-default renderViewportScale)

There are 2 different costs to consider here:
1) The number of pixels being rendered and texels being read. This is related to renderScale * renderViewportScale. So 2 * 0.5 = 1 should have no impact on this.
2) The memory footprint. This is controlled by renderScale and has nothing to do with renderViewportScale.

Sounds good. In our case, increased mem footprint should worth the improved visual quality.

Thanks again!