Hi NEO-N,
Not sure what GPU renderer you used was but Redshift is different from all the other GPU renderers. You said the GPU renderer you used (not Redshift) you was limited to 15M polys, that's horrible, or your graphics card was junk. You can fit 15M polys on a card with 1.5gb VRam easy, think that's a $80 video card. I can tell you from one of my projects that was around 60M polys we rendered just fine and unbelievably fast on a $300 GTX 980ti with 6GB Vram, no problem. That was pure polys before any instancing mind you. You say Corona is unbiased, you mean Pathtracing + Pathtracing, doesn't most use Pathtracing + HD Cache cause it looks pretty much identical but much faster to render? I mean in Redshift you can also do Brute Force + Brute Force if you want an (un)biased rendering, but BF + IPC is like Corona PT + HDC, and output is identical to Corona. You even have more options In Redshift for Biased rendering, but you don't really need it since its so fast. A lot of people try Redshift and don't know what there doing, I know when I first tried I was using it wrong, I thought the progressive renderer was the final Renderer and even though it was still fast and good, I found out after bucketing rendering is where all the magic happens. We recently did a animation and I tried Corona 1.7 for one of the rooms to see the outcome, did a great job, but you couldn't tell the difference from the Redshift render. I think there was one issue with the shader or something, not sure if it was the shader or core, but from using shaders from various renderers I can say by far Redshift Material is the most advanced material shader I have used.
So I see a lot worried about vram and scene size, I was too at first until I realized how huge a scene Redshift can actually render with no issues. Other GPU renderers I definitely had issues with even much smaller scenes. Soon we will also have NVlink which will share memory with cards, then Vram issues will be gone for even the most extreme cases.. For now those are only with the highest Teslas I think.