Ello:
This might work in the case of a terrain with only one tiled texture like grass. But I can also map a sci-fi room with lots of tilable textures, many floor tiles, ceiling tiles, wall plates and columns with special textures. After that I create a second uv-set with a none-tilable uv-map and bake the shadows / static lighting into it. I can use even GI, Radiosity, area lights whatever I need.
At the end I combine both textures and uv-sets and could even live with 2 medium sized textures like 1024x1024.

Such a level could look beautiful and would render full speed. My only concern is how can I add normal-mapping to this scenario. If anybody finds a solution then this will be my favourite tool.

I still did not find the time to check this out. Have to finish some paid jobs. But I am still highly interested in that.