I received my HD131Xe this week from Amazon. I don't know if I have the focus problem or not. I mean, my focus is "good" in all 4 corners but the focus isn't exactly the same in all 4 corners. If I focus the upper left to be razor sharp, the lower right will still be in focus but not razor sharp. It's still OK, certainly not dull or blurry, but it's not exactly the same sharpness sharp as the upper left. If I was to compare, I'd say upper left is 100% sharp and lower right is 85% or something like that. Something I can live with and probably a drawback of a projector in this price range? Not sure. Anyway, if they're supposed to match, though, like 100% sharpness in all four corners, I might RMA, but then that might be being petty, I don't know.
Side note: I'm using max zoom and adjusted the focus as best as I could.
From your description (100% sharpness in one corner, 85% in the opposite) I'd say your unit is well within acceptable tolerances for a $750 projector.
How does yours compare to the screenshots (click on them to see in higher res) of mine in post 879? Really rough guesstimate, but I'd say the corners on mine (110" screen, no keystone) are within 10% of each other with the top center area being the least sharp but only noticeable if examining at less than 5'.