You can swap barrels just as easily on a Defiance as you can an Impact. The advantage of an Impact is that the tolerances are held so tight, that a gunsmith doesn't need to see the action to spin you up a shouldered barrel. With a Defiance, they need to see it once, and as long as they save your action spec's, they can spin you up further shouldered barrels without seeing your action.
Another option I would strongly consider is the Lone Peak Fuzion. It's not as hyped up as the Impact, but it's every bit as good (if not better). The newest generation of the Fuzions are held to such tight tolerances, that a gunsmith never has to see the action (like the Impact). It runs great dirty or dry, and once broken in, that action feels butter smooth. In fact, pretty much everyone that I've talked to that has had both an Impact and a Lone Peak Fuzion has preferred the Lone Peak, including two top 10 PRS gunsmiths. No doubt the Impact is a great action, but it's certainly been hyped up hard on the internet - to the point where people are buying them and are left wondering what all the hype is about. I've heard it a few times.
All great options, but if I was in the market for another push feed action, it would be the LP Fuzion. Just got one for my wife's rifle build, and it's pretty slick. But honestly, all the top tier actions are great. Once you get into this tier, it's just nitpicking on your prefences and critiques between the actions.
