All the posts I have seen talking about specs are talking about where they will be with the orange meta.
How about this? I'm sure there are people like me who aren't at 500ilvl and are semi-casual raiders but still want to be the best they can be for their team.
I'm personally only at 485ilvl (yeah, I know, terrible luck with drops), and my "4 hours a week" group is working its way through HoF and into ToES.
From what I've seen, people are saying DemoSup/Serv > AffSup > DestroSup is the order of effectiveness, but is this true throughout the iLvL range, or just for top end HM raiders?
As a side note, I looked at the simulationcraft website for a new build to try and answer this question myself, but the builds there are still 5.1, hence my request from the wise warlocks on this board.
On that level you really should just stick to what you are most comfortable with. You will do far better with it then another spec even if it is rated higher.
That said, I suspect for your level it will indeed be Demo>Affli>Destro by small margins between them, for single target. Once you start factoring in multi dotting and AoE, you know, real fight mechanics, things start to get more cloudy. I've so far heard most raiders saying they will go for Demo or Affliction depending on the fight, not depending on how strong the spec is, but on how well the spec fits the fight.
You can use the last 510 SimC release with PTR settings to get a feeling for what your DPS will be like. And there will be a 5.2 release "shortly".
T15N profile won't have the legendary meta gem, so that should help you to compare specs. In any case, Input settings like gear ( including meta gems ), talents, action lists, glyphs aren't part of the core Simulation. They are always completely at the disposal of the user, so you can set up profiles in any way you like and simulate them. Don't count on SimC to always provide profiles for you. And even if there are profiles, they are just sample profiles and never Best in Slot ( even though they may aspire to be so ).
I have almost the same itemlevel (486) and ama casual raider myself. I tried all three specs (at least on a dummy) but in the end I decided to stay Destro, since it's my favorite by a very very long margin. I did better in it than in the others, as xskarma said experience in a spec can to a certain degree outweight the mumerical advantage of other specs.
I completely agree with what xskarma and the others have said. Your playstyle comfortability will increase your dps more than any FOTM spec choice. I am also a fairly casual raider and I have noticed at 490~ ilvl, Aff is definitely not hitting like it used to, to be expected, and demo is pulling much larger numbers then before. Just my personal experience but if your comfortable with demo I suggest giving it a shot.
I just came back since before christmas. I was playing demo/aff. But just doing some quick work on the dummy with like 475 gear still reforged for mastery, I was doing a few k more DPS as dest then as demo. I switched to dest because I like the play style a little more and well, green fire.
Demo/Aff covers most fights pretty well since Demo can more easily spread its damage to fights with more than 4 targets or in cases where Aff will easily get shard starved. They're competitive enough too at the moment that you should be fine with either, whichever you're most comfortable with.