I didn’t remember having trouble with purple corners with the NEX-5, so I thought I’d run a test. I took photographs of a cloudy sky with both the NEX-5 and the NEX-7. I used my worst lens for generating purple corners, the Leica Elmar f/3.8 24mm. I set it to its worst aperture for purple corners, wide open.
Here’s what I got with the NEX-7:
And with the NEX-5:
The NEX-5 is not completely free of color casts as you go away from the center of the picture, but it’s a world away from the NEX-7.