For as often as you update the code, it's a fair burden to re-download the 100+MB each time. You could solve this by putting the graphics in a library that doesn't need to get updated every time you have a change to make.

I wouldn't mind but there are a lot of people that wouldn't like this. I still get asked to put the hr graphics in the main mod even though it's impossible because of the portal filesize limitation.

When author will wish to do masks instead textures then SR and HR graphics will be present in single mod and whole mod size will be less than 50MB.

Unless factorio changes the way tile graphics work that is not possible.

Is any chance that Factorio developers will hear us with this suggestion?

Since it would impact all other mods/devs it likely won't be changed. It isn't too bad and it means people with lower end machines can skip it.

They might consider allowing 2-layer tile textures. What I really want though, is to be able to change two layers of tint on a per-tile basis, not just per tile type. That way you'd could gradually transition grass colours using noise functions.

That's a great idea, should help deal with both limits on loaded tiles and overall performance and file size by compositing two together. Noise functions between biomes would make transitions prettier!