Can you elaborate further on what you mean by barreling side to side? A K2 frame is pretty much impossible to "barrel" the frame shape and cause discomfort with heavy loads. Do you mean it just moves more side to side (when you lean left and right while wearing the pack) then you would like under heavy loads?
That was a rare complaint of the K2 frame but one I would see pop up from time to time. It was something I wanted to see if I could fix in the new frame but do it without sacrificing performance in other areas. For example not reducing the lateral movement that Sneaky has mentioned, in his opinion, that we lost too much of in the K2 frame.
In the new K3 frame I ended up designing it with a rigid foam structured "wing" that extends out into the foam of the hipbelt on both sides. This does definitely reduce that side to side movement that you aren't liking as well as really reduces any sagging issues for guys who don't have a lumbar curve (i.e. they have no butt). The shoulder harness webbing is also sewn to these wings so they sit wider out on the structure and aid in stability.
As far as X pac goes we have no plans to use that in the packs anytime soon. We used VX-21 Xpac in our original 2014 packs for the inside of the bag. After a year or so of use the waterproof layer really broke down and it was substantially worse than just using Cordura with a PU coating in regards to water getting through. On the heavier weight Xpac options that might not be as much of an issue or an issue at all. To be honest we really haven't revisited X pac since after that experience. It's also, in our opinion it's just too noisy of a fabric for the outside of the main bag, but we understand that's definitely a personal preference for guys. A lot of that depends on where you hunt from a brush and moisture perspective as well as just the features you want in a bag. Exo is a small company so we really focus on keeping things as simple and streamlined as we possibly can. 500d has been an all around outstanding performing fabric for us.
Steve