I ski the BD Havoc w/ AT setup. Haven't skied the Jak or the Jak BC, and don't know anyone who has. From what I've read in comparisons/reviews, the Jak BC would be a softer ski longitudinally. The Havoc is fairly stiff longitudinally.
The Havoc skis LONG. I have the 173cm models, and compared to my Fischer Big Stix 8.6 in 180cm, there's almost no loss of stability at high speeds on groomers or hardpack. However, the Havoc doesn't have the same damp yet powerful feel of the Fischers. I guess that's the tradeoff when the ski is designed for backcountry AT/tele use, they keep it light by losing the heavy metal or synthetic damping sheets.
I like the Havoc, but didn't ski it very well until I figured that my lack of binding delta on the Naxo AT was causing me to ski backseat relative to my alpine ski setups. Once I shimmed my heels, the skis came alive.