Couple of thoughts.

It's hard to predict the results. I don't think the 3D Coar paint room is set up for models with that many ploys, though it may be worth trying. Also, I don't know that you need to use 8K maps for a large scene.

MeshLab has some great poly reduction algorithms, though I suppose the best way to reduce ploys is in MoI.

I suspect most 3D paint programs will choke on 50 million polys. What is/are the model(s) you're working with?