Faulty logic. One of the biggest reasons for supporting liberalized immigration is that it lets people move to the areas where they are most productive. People in Guatemala become massively more productive when they move to the USA, which earns them higher wages and makes them better off (along with others who benefit from more production).
This website itself endorses that reasoning:
http://openborders.info/place-premium/
What you're talking about is the more limited concept of "brain drain". IMO its practical importance is a lot more questionable than people think. It's a small factor compared to the global benefits of liberalizing migration.
And why do you think that? Is it because we already have a lot of research already on this topic, or do you think good data from research is unnecessary?
It's not really an issue that warrants a lot of study, in my view. If the Guatemalan government is happy to see its peasants swarm to the United States, they can probably be trusted as authorities on the value of said peasants. A humanitarian could only say "These people are beneficial!" than he would immediately realize, "And Guatemala needs them more than us. Send them back." That has never happened.