"do/can/will/plausible that FMJ rounds out of a 5.56 will penetrate armor?"
Like any center-fire rifle cartridge, 5.56 mm FMJ will penetrate all soft armor (ie. IIIa, II, IIa), but will not penetrate hard level III and IV armor any better than 5.56 mm OTM, PT, JHP, or JSP ammo.

"When a M4 style weapon is your primary defensive weapon is there a round that you like best for the firearm?"
https://www.m4carbine.net/showthread.php?t=19881

"Is there any one brand of body armor you see that regularly out performs competitors?"
For soft armor: https://www.m4carbine.net/showthread.php?t=19912; for hard armor: https://www.m4carbine.net/showthread.php?t=19910.