As for the claim that National courts don't enforce Nafta, what are all those lawsuits [chapters 6, 7, 11 below] doing in US and Canadian courts?
http://www-tech.mit.edu/Bulletins/nafta.html
These treaties are litigatory boondoggles for trade lawyers; nothing like expanding market opportunities in rent seeking, what with zero-sum games in oligopolistic markets becoming increasingly likely and the possibility of stagflation always around the corner.
The core problem isn't sovereignty; it's the transcendence of democratic accountability and representation and the balance of interests tests that have sustained federalist approaches to economic governance that is at issue with these new regional and global economic "constitutions".
BTW it's state and local governments that really get the shaft with these agreements.
See volume 31 number 3 Cornell International Law Journal: "Sovereignty by Subtraction: The Multilateral Agreement on Investment" by Robert Stumberg.
Ian