It depends on what "net positive for the US" means. There would be immediate product and service shortages and lost intellectual capital, but a huge explosion in available housing, spots at universities, open jobs desperate for labor, etc.

I use the dramatic case of "all immigrants going home" to illustrate that immigration affects the housing crunch and it's ridiculous to talk about it without mentioning the population increase. Personally I think the government should intensely crack down on illegal immigration, destroy business that use illegal labor (even if that's to the detriment of the consumer), and then make legal immigration tuned to attract high-skill immigrants easier.