I bit the bullet and gave it a shot. I am happy to report that the firmware upgrade was successful using Kies, I was able to root the device via the Kingo Root app, and was able to re-install TWRP. I am now on firmware M819NUVUAMK7. It is still Android version 4.2.2, and the baseband is also M819NUVUAMK7. The kernel version is dated Fri Nov 22, 2013
I installed superSU and when I ran it, it asked me if I wanted to disable knox (YES OR NO). OF COURSE I DISABLED KNOX!!
I afterward performed a nandroid backup in TWRP (the first order of business).
I am more comfortable now deleting, freezing, uninstalling system apps now because (in the event of a softbrick or bootloop) I can always revert to my nandroid backup or in extreme cases, restart all over again by re-installing the firmware via Kies, re-rooting the device via the kingo root app, and reinstalling TWRP via ODIN.
Not sure what changed from the original firmware (no change log). In any event, the device is snappy and I'm not experiencing any lags.