Also, the only thing you can really do that can set you apart from other spriests ( except proper positioning ), defensive wise, is proper guise usage. Most cleaves will either have shitloads of small DoTs on you, rendering a guise longer than 0.5 sec impossible ( and shielding then guising usually doesn't work seeing they chew through the shield during the global you've put it up ), or they'll instantly cleave the clone down - it's your job to judge when guising is more beneficial ( when a snare is about to drop, guise to drop target, create distance/LoS without snares on you ), or when a mage gets some CC going on both melees - guise without it instantly breaking, etc.
But yeah, I'd say Godcomp is 90% in hands of your Mage+Druid, they need to be experienced with the comp, you're basically just their 3rd doing as much DPS as you can and initiating CC when they both can't start one, or following up CC when their stuff is exhausted.
They drastically reduced the ability for one player to carry the team, and made it more about teamwork ( both a good and bad thing ), Spriests were hit extra hard by this. The most skillful things you can do as a Spriest atm are things that used to be considered normal play for even Duelist players in WOTLK/Cata ( predispelling with mass dispel ( both offensive and defensive ), landing 2+ person fears, etc ). Spriest in Godcomp can be a 3k player and be stuck on xxxx rating with improper mage+druid, and a xxxx rated spriest can be a 3k player if playing with a 3k mage and druid. It's kinda sad, but at least we have a tier 1 comp atm so /shrug.
Work on your synergy and work on getting as much 3 people CC as you can during your kill attempts and you'll considerably rise in rating when you start being dicks to people ( read : entire team being controlled while one of them dies equals whining on forums ).
Also, protip when you're getting trained - if you can't get Vampiric Touch up, don't bother getting SWP up. MBing on CD with VT+SWP is your highest "passive damage" ( when you aren't actively using flay, spike, devouring plague ), MBing on CD with only SWP up is about 5% less damage than MBing on CD without SWP up.