I can't tell you how they ride compared to Harshmellows, because I haven't ridden a k2 binding in years.
Our board designer here has the worst knees. He's getting knee replacements in a month or so. He's been riding the Factory and loves it, mostly because of the canting.
Stiffness: The highback is pretty damn stiff, but the base has some flex. It's somewhere between Charger base, and Atlas base. Those are pigment free, or 'raw' injection. This makes it a bit more stiff. It seems to be our most wanted binding right now with the group of people we flow stuff to. I think it's because the Factory is an upgraded version of the team bindings we've been making for the past 2 seasons.
Dampening: Yes. Again, I haven't ridden K2's, but I would guess they would have similar dampening.
I hope that helped. I'm in a rush right now, sorry for any type-ohs.
Are the Factory's the SL replacement? I didn't see (but might have missed) the SL's in the 14 catalog.
I will admit that after some initial challenges with the toe strap (just didn't work with my Salomons) that I really... really like my SL's. I haven't rode my Restricted Cartels but once this season and am thinking about selling them since I don't see myself riding them much.
Opposed to some on the forum... I'm actually really impressed with how durable the SL's have been. The only paint chips I have are where the forward lean adjuster meets the heel cup. Pretty impressive!!!!
If the toe cap is improved to work with more boots, I really wouldn't be able to think of a reason to not recommend Union's to people I ride with.