I went to SIS, and unfortunately I can't really disagree. I did alright on the job market, but I had my active security clearance to thank for that. In all my paid and unpaid experience, I think I've only run into one other SIS grad (and I'm not 100% about even that one). You wouldn't think one ranking spot would make much of a difference, but the drop off does indeed seem to be pretty steep. I fear I'm something of a permanent second class citizen in IR/the Beltway on account of it, to be perfectly blunt.
That said, the causality isn't completely obvious. I'm not sure how much SIS grads struggle because they're SIS grads, rather than struggling because they were in a worse position before even starting grad school. After all, professional schools love to select students that are already on track so they can claim implicit credit for their success. Moreover, it's my sense that school prestige matters little in government (including government contracting), so it's mainly consulting and the non-profit world where you're going to have a problem. These are probably better jobs in reality anyway, even if they're not as glamorous. On the other hand, if you're dealing with a sector where prestige is immaterial, then there's probably not much reason to shell out extra for a pricey private school (though the location is still a significant benefit).
As an aside, GW is kind of in a gray area, almost a class unto itself -- I wouldn't put it on par with the higher six personally.