Seriously apple needs to Quality check their products before they rush out the door. This image retention issue + the gamut-gate issue = terrible iPad mini 2. For sure iPad mini 3 will fix these issues.
The gamut issue is a design issue (the displays were designed with the lower gamut) and not a production issue (you don't get a gamut variance between different mini 2 units). The image retention is most likely a production issue from the vendor they get the displays from. Since it appears to differ from unit to unit, nobody knows how prevalent the issue is. In any case, just exchange your mini 2 if you think you have the image retention issue.
I would say the mini is a very good tablet if the lower gamut isn't an issue.