It could be profound --- but that doesn't mean bad --- what would happen? The fallout from open borders is that the corruption in Mexico continues, laws don't have to change, they continue with their two-tiered society. If the border were to close up, there would be pressure in Mexico for some badly needed changes, a push to allow a middle class in that country. If the borders were to suddenly be controlled, the elites in power would likely immediately pack their bags and head to exile in Spain --- the most corrupt would be the first to go.
You're right. A closed border would definitely mean no more US safety valve for Mexico's unhappy lower class. Pressure would certainly build to change society. It could be the making of that country. The oligarchy would cease to exist, the corrupt and powerful would eventually flee and a middle class would come into being. I hadn't considered the benefits for Mexico at all. I had blinders on and was only seeing things from a very morose US perspective. Funny, in all the eloquent essays on the Mexican/US border woes by V.D. Hanson that I've been lucky enough to read, I don't believe he's discussed it. He's had to have touched on it, of course, since he's an historian and a native Californian...and it's so clearly in Mexico's interest for this to occur.
Speaking of a nascent Mexican middle class, that must have been the prize in their NAFTA crackerjack box. Perhaps it will happen, but more slowly than we'd like.