Native res probably is under 30 FPS for D3. That's completely unplayable for me, maybe not for you.
Some people don't really see the tearing or are bothered by it.
Yeah, could be my eyes. I only see tearing on high frame rates.
Native res with shadows off goes at 36-37 during light combat in Act 1 inferno. Dropping to 1536 pushes it up to 57-61 fps which is quite enjoyable, but anything over 25 is playable for me, considering I had to play competitive gaming back in the days with 200+ ping and low end components, I guess i'm just used to being able to play on any setup.