Hmm, from what I have heard (haven't tried older 390's), the 2011 version has stiffened up a bit. I used the boss in some great pow runs and it did a pretty good job. I feel that if my boots were stiffer, it would have fared much better.
It's pretty dead center of medium flex spectrum in my opinion. Great for my style: I dabble in everything I can (minus jibs).
It's hard to recall whether or not the Uprise was stiffer than the Boss. I only recently picked up the Boss and I rode the Uprise last Feb. I do remember it being a mid-flex binding as well.
To the best of my memory, I would say:
Boss: 5 flex