My opinion is that it is somewhat competitive, but not as much so as getting a GI spot itself. It's not a sure thing for one to match in GI, even occasionally for those from top IM programs. OTOH, you should be able to get an advanced endoscopy spot SOMEWHERE in the country if you are willing to move. A spot at Hopkins or Partners remains very selective, of course.
One thing for sure, advanced endoscopy is definitely more competitive than hepatology.