What TV do you have? I've seen 720 TVs recognized by my Roku Stick as 1080, and while it worked it wasn't the native TV resolution. Just because a TV accepts a 1080 input doesn't mean that's the actual resolution. It could be your TV really is 720. But a make/model would help confirm or disprove that idea.
Found an online review of the PN51F4500, which I believe is the full model number of your set. Sorry, it's 720p native resolution, not 1080. http://www.hdtvuniverse.com/plasma-tv-reviews/samsung-pn51f4500-review/
I think you're best bet is to remain set at 720p. I think the quote you have there is for the more recent LED 5 Series sets, not your older plasma.