The Heritage Foundation doesn't exist to research credible conservative alternatives. It exists to provide pseudo-intellectual claptrap to paste a fig leaf over the nasty bits of wingnut prejudice. Heritage proposed Obamacare in the 1990s, only to go full metal jacket on it when a Democrat proposed it.
Same on immigration:
[T]he study, which comes out under the leadership of conservative former Sen. Jim DeMint (R-SC), is a sharp departure from a “Backgrounder” the Foundation published in 2006. Then, Heritage noted that “worker migration is a net plus economically.”The wingnut bullshit artists are in charge, and they need to keep the rubes inflamed so that they don't notice it's the plutocracy who are screwing them.
Update: My man Paul Krugman agrees with me. Usually it's the other way around.
The truth is that Heritage has never been in the business of doing economic analysis; it’s just a propaganda agency posing as a think tank.