Vicious Syndicate put Highlander Priest in tier 4.  However, I played their featured list with only one card changed 21 times and won 18 of those, I faced a lot of aggro demon hunters and somehow won against all but one of them.  They didn't seem to make any misplays.  This deck also won against other decks labeled as tier 1 in vicious syndicate like pure paladin, evolve shaman, and highlander hunter.  This was in platinum, I haven't played standard for a long time, but since I was wild legend consistently probably my mmr would be high for standard as well.  So, overall, even if the deck isn't tier 1 I think that if people play a good version of Highlander priest and play it well it is definitely higher than tier 4 and may even be as high as tier 2.  

If you have played Highlander Priest in standard recently, or can try to play it to have a higher sample size so we can see if it's really underrated or not, or just have some thoughts please let me know because I usually play wild only but based on the vicious syndicate standard report I thought this might be interesting to people who want to play some other decks than demon hunter, shaman, paladin, and hunter.  Vicious syndicate puts Highlander Priest in tier 4 and calls the class a trash tier class with no good decks but please give it a chance, because I'm not even that familiar with the standard meta so someone who knew the opponent's decks better could have had an even higher record with this deck.  And most of all, out of all the standard decks I have tried to play so far this is by far the most fun and engaging deck to play, you plan ahead your turns and make decisions in a fun way, in my opinion at least.