Herra: I refer to the texture before being mapped-to-sphere being 2k. I'm not sure what "planet bitmap" means exactly, could you explain? The width of the planet itself after being mapped is 1448 pixels (1500 with atmosphere) if that's what you were looking for.
Also I think I'm letting my personal preference of having thick/obscuring atmospheres sneak in there again. I can make a brighter/clearer one as well.
By planet bitmap I mean the end result, ie. a round image of the planet. The file that can be used by FS2 as a background image.

Anyhow, if the width of the texture is only 2000 pixels and you wrap it over a sphere, you only get 1000 pixels for a side, and the central area facing the viewpoint is stretched even more, so having the planet 1500 pixels wide will probably show some texture stretching and would probably looks better at half a size (750 pixels wide, the whole bitmap being 1024^2). But as I haven't seen the high-res versions, I can't really say anything certain about it.
Also like I said, it's good practice to use power of two textures. It hardly matters with GIMP (though it well might, who knows...) but it's a good habit to have.


^
TopAce: The last one is not actually better or worse in itself, but it differs from the above image more than the former version, where there was more contrast between surface features... so for the purpose of replacing that in the game, the former one is, in my opinion, somewhat better.