There does look an inconsitency with the coastlines. At first glance you can't see it, but then it can look 'odd'

Is this RobA's tutorial you mention? You could work at a higher res than needed and then reduce, which will give you detail.

I did a tutorial for Photoshop which is an alternative technique, which might help. RobA told me about his tutorial after I did mine, I'm looking into adding it in. We discussed it a bit here

Map's looking good BTW