Barrel harmonics degrading accuracy is so miniscule it isn't even worth mentioning. About the same as the accuracy differences between free float and non free float hand guards. No doubt that it exists, but the myriad of human variables would completely trump the aforementioned issues. To me, the potential for parts failure due to lack of standardization of piston systems is far more of a concern than accuracy loss.

Anyone have experience with .308 AR's? Are the magazines interchangeable between manufacturers like they are with 5.56?

Yes and no. Armalite and its clones use a different mag than the lr308 (ar10 is armalite brand, some have copied the standard lr308 is everthing else) some manufacturers use one style, some use the other.

My build is a lr308. Brand preference. The dpms uses the lr308 mags. If you buy production, just pick what you like. If you build yourself, find your upper/lower first, then make sure you get the right mags and barrel ext. Everything else should be same. The barrel ext may be same, its been 3 years since I ordered mine, so I brain dumped that info. If you want to build one, lemme know and ill give you my number and ill share resources and info so you can decide what you want. Ive got it all documented on my work pc.

Ran a few hundred rounds through mine this morning. For what I do, it fits the bill and made my 1k budget.
http://youtu.be/adlEEi2rN9I