In fact, the 1600x1200 I mentioned +is+ ~254PPI which would be just about perfect for this device ... though still subject to the major downside of app compatibility - plus factor in the discussion about the ever growing app sizes, I don't think we need a 3rd set of iPad assets in an app!
They could just scale down the @2x iPad assets in current retina iPad apps to maintain compatibility. Then they would just need to add support for a splash screen image (since those need to be exact sized as they aren't processed at launch time) - so something like a Default-800h@2x.png image.
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Technically possible, but software wouldn't run unchanged. Worse, unlike the iPhone 5 where old software runs just fine with a bit of a black gap at the top and bottom, this would give you fewer points, so without any changes many old apps would cut off stuff at the sides. Quite horrible.
Why would the sides be cut off? Both the current iPad and iPad mini are 4:3, so the ratios are the same. They could just downscale the current @2x assets and it would fit directly. And iOS uses the non-retina values for touch points, so those would be unaffected.