Here's a video of an 1800, but maybe it doesn't count because it happens during a triple back flip.
Of course in freeride lingo (adding 360 every time one goes inverted) maybe it would be called a 2880.
No, it's not grabbed, not even close, but if guys can do this, then I would have to say that yes, 1620 is possible.
http://www.youtube.com/watch?v=jo5WAYI4RFk
I'm not a huge fan of aerials, but it's still pretty impressive that someone can do that and land clean,