I authored the post to which you linked. I keep a list of recommended OS X software. This is a general use list, but some of it is decent for mapmaking.

In my experience, GIMP on OS X doesn't suck any more than it does on other platforms (but, then, I think GIMP is mostly horrible interface-wise anyway, so take that with a grain of salt).

I haven't updated my list in years. One thing I should add is Pixelmator, which is inexpensive and quite nice.

Also, keep your eyes open for bundles like MacHeist or theMacSale. Pixelmator, for example, is sometimes offered in bundles like this.