This peice of armor runs on he same concept as the Devotee's Mitts, you need the Zealot's Mitts to synth them, they have higher stats than Zealot's mitts, but in no way is it a HQ synth, due to the fact that you're not making Zealot's Mitts, you're making Devotee's. Same applies here....just umm...replace Zealot's Mitts for Priest Robe and Devotee's Mitts for Bishop's Robe
There is in fact a +1. There is one for sale in the Jeuno AH in Pandemonium, but it is over 200001g, so I don't have it. I refuse to pay over 4x the original for a +1. Anyway, Same robe, just add 1 to all the stats.
+1 has a white border Conquest synthed upgrade has a white border (Bastokan or +1) Conquest synthed upgrade +1 has a blue border (Republic or +2) RSE has full grey background
Stackable items can HQ into +1, +2 and +3, although the stacksize is the limit. Shihei have +2 as a max (99 instead of 33) but log->lumber has +3 as a max (4 lumber as max, 12 if using *Lumberjack)
there is no +3 (except in the case of stackable items) otherwise you are right, but that pic shows the standard background :P the +1 background is much more striking than that and you can't see the bevel that's in this border