How are you moving kids forward using KHAN when they are academically not any where near grade level?

I have been looking at the research on KHAN and have found the two camps. Its great "Yay, us!" and "It doesn't make any impact on my students scores."  What I've found, or at least what I've read so far in nay sayers, is schools and teachers using it to supplement what the kid is already "struggling" with.  Struggling is a gigantic catch all!  For example, if they are struggling with a concept in fifth grade they get fifth grade level KHAN assignments to address (aka fix) the problem.  This might work with kids that need some "polishing" of skills, but not with kids that are significantly below grade level.

I teach in a school that has 97% free and reduced lunch.  The community struggles on so many levels.  What I do in my 5th grade class and others here at our elementary school is have the kids start from early math and prove themselves up to grade level.  If its too easy, "Good for you! Keep going!"  Kids that know everything early math has to offer should fly through it in a matter of a few dedicated days. Same with 3rd grade, and fourth, but what I've found is most kids in my 5th grade class struggle to get through early math and third grade through out the year.  Very few get through 4th grade let alone successfully master fifth! 

If students are demonstrating such difficulty proving mastery skills their grade placement says they should have mastered, "You're ten so you are obviously fifth grader," why are so many research papers and schools trying to get results forcing kids to practice grade level lessons when they are academically no where near grade level?

How are you moving kids forward using KHAN when they are academically no where near grade level?