I can't help with the data, my laptop can't handle it.
But I can give some advice regarding image processing:
» Do a full resolution luminance mosaic
» Do a 1/3 or 1/4 resolution color mosaic with consistent results in mind. Using band X or Y will not matter much because the actual ground color will change with seasons, atmospheric dust/water vapour, etc, etc.
» Merge both as a final step
Dont' know if this will speed anything up, but I guess that working with larger areas might improve color stability.
Now, an even better approach would be to load the data separately.
Use a global lower resolution color map, and then generate the high resolution shadings using the height data and normal maps.
And of course, a specmap to give the albedo variations.
This setup could handle seasonal variations and might be easier to assemble.
Finally, if you simply correct a satellite image to match the colors seen at the ground, it will not work, because the light is completely different. From Orbit you have 2x atmospheric effects, from the ground only 1x. That's why the images are always so blue.
Sorry for rambling about so many things, but it might be helpful, because I've worked with satellite images many years ago and actually calculated atmospheric corrections by chanel, season, hour, etc, etc. It aint worth the effort, simply eyeball it
Just my :2cents: