I get where you are coming from concerning performance issues. My current map is pretty much the same size and the working file is about 900MB - it takes several minutes to save it, and 20 seconds to just move a layer around. When I had hardware issues that reduced my RAM to 4GB, I couldn't even work at the map at all. I certainly learned my lesson.

I am still too unexperienced to give you any artistic suggestions, it looks great! But the nation borders seem so uniform with the shape of the continents if you know what I mean. Are there any territories on foreign continents that are held by another nation (like the Str. of Gibraltar). Some remnant of a past war... Oh nevermind, I just realized that the colors are continental borders, not national ones.

What is the difference between the thick borders between nations like Tenhares and Mysaroh, and those thin borders between other nations?

Oh, my favourite suggestion: It is "known to mAn" (unless your women know much more about your world)