Just an update so far:
Spoke with a senior advisor over the phone with Apple last Friday. He advised me not to go to a store and they want to take care of the issue over the phone since I don't have a store within any close proximity. They requested I clean install High Sierra on my partner's 21in iMac as I did with my own 27in iMac last week, before they would proceed further with any other options.
Tonight with both iMacs on my same desk, both plugged into the same APC UPS and hard wired to my router, I shot a video of both from a cold start. Just as I wrote here last week, 15 seconds to the login screen on my 27in iMac. Hers booted at 49 seconds to the login screen and another 13 seconds for the mouse to even appear on the screen. I just sent the video to Apple as well.
Edit:
Here's my desk at the moment...
i have tested my imac base line.. 1minute 50 second .Boot.. If you insist need super speed go to SSD.. return and ask upgrade.. If normal usage the most advantages just copy file just that.. Are it worth to spend upgrade 300 dollar ..That's you choice.