It due to the realy poor quality of source files the gold saucer and some other files are realy low in quality and upscale them is a choise between heavy noise + dithering or heavy filtering..
In the future v4, it'll be better (not going to do a miracle ^^' but will be better)
Check the sbwy4_5_0, maybe the lowest quality in original game, haven't found a way to process it ATM