This has been fixed by a reddit user. The fix is documented here, I can confirm it worked perfectly on my 2014 27" 5K iMac with the original LG panel and severe ghosting effect, now gone:
“Hello,
Firstly, I know this is an old issue that most probably does not affect the majority of current Macbook users. However, I thought I would post here just in case this helps someone, as a current MBPR 2012 owner this issue has been driving me crazy but I've finally figured out a way to stop it from happening. And it's really simple:
Go to System Preferences -> Displays -> (Built-in Retina Display) Colour tab -> Calibrate...
In Calibration, drag the white point slider to the far right (most blue light). This causes the screen to become extremely blue, so to cancel this out:
Select Night shift, adjust warmth to preferred level (far right for best results), and set up the night shift schedule to always be on.
Essentially what this does is makes the display dimmer by applying two filters that counter act each other. As a result, the pixels themselves emit less 'light' or colour, and are not driven to the point at which colour retention occurs.
I hope this helps my fellow LG screen MBPR 2012 users out there. This machine is still a beast in many ways and deserves to be treated as such. Shame that Apple won't fix what is essentially their production fault on a laptop that RRPd for £2000.
Thank you for reading!”
