What is North America seems to vary on geopolitical bases. As for birders, and specifically at this forum, I think we treat NA basically as the ABA area. I am perfectly fine with this forum being anything not in the ABA.
I think it would be much better to place birds from Mexico and Costa Rica in this forum. There really aren't that many of those posts, so I don't think that Costa Rica posts are swamping out the rest, at least certainly not to the same degree that the Costa Rica posts would surely be swamped out of the main identification forum. Given that many of the US based birders are nearly as familiar with birds outside the ABA, it can take a while to get a response to a non-ABA post, and by that point the post would no longer be one the first page and may never be seen. Whereas on this page, you'll see that the first page of this forum still goes back to mid February, so certainly CR isn't swamping out all other posts.