This is an interesting thread. Thanks for posting it.
I am going to refer you to this gao site. The gao criticised the SS Aministration for only spending 2 days in Mexico looking at Mexico's ss system before signing this deal. If you will scroll down to page20 of 35 you will see the pros and cons by the gao and ss administration. It also gives reason and rational for these agreements. Page 26 og 35 lists some of the reason and one being the input of the USTR depart or trade relations.
There is also a table that tells how the previous countries have been either over or under estimated. Some of the countries underestimated have been under more than a 100%. Big mistake that we can't afford to make with Mexico as it will be the largest country and even the gao admits not knowing the number of "unaurthorised" workers and their dependents.
http://www.gao.gov/new.items/d03993.pdf
If you can't read it now, read it later.
For some reason you didn't get pinged for post 231. You might find this link interesting.