I can't help with that particular comparison, but I do use a 5k 27" iMac. I calibrate it with a Spyder 5. There was no issue with going from my older Apple Cinema 30" screen to the iMac, but I wish the iMac was a 30" screen. No problems at all to produce prints from my Epson P80 and P6000, and equally no problems outputting cmyk files for press production.
I think that a good screen, properly calibrated, is important, but that there is also a bit of hype regarding super high-end monitors. The iMac may not be the ultimate monitor but it sure is up to the job.
Jean-Michel