Since Blizzard simply changed global dps modifiers the numbers are pretty straight forward 4% for assassination, +8% for subtlety. What this means and why it happened is another question.
The assassination nerfs make sense on their own, remember assassination came into this tier at approximately +3% because of the dispatch and mutilate buffs. Many rogues (myself included) wondered if those buffs were really needed as assassination didn’t seem to need single target dps buffs. In that context these nerfs make a ton of sense, we the playerbase were correct assassination didn’t need those nerfs and Blizzard has come around to that conclusion.
Where things make less sense is with respect to spec balance. Assassination is a very strong single target spec but so is combat with Assurance of Consequence(AoC). Combat probably does lose more dps on interrupted damage fights but the fights where assassination was clearly over the top were pure, uninterrupted single target fights, Norushan, Juggernaut, Malkorak and Siegecrafter. Combat and assassination were very well balanced this tier, combat has cleave, assassination has execute, combat has regular burst, assassination handles interruptions better. This change undermines that, nerfing assassination without nerfing combat means that combat with AoC is probably the stronger single target dps spec. Related to my earlier point since Blizzard buffed dispatch and then nerfed damage across the board assassination is now a weaker spec for non-execute fights, how much this matters is a bit unclear but its worth bringing up.
You don’t see combat tearing it up on Raidbots because there are just a lot fewer combat rogues out there. There are a few reasons for this, first a lot of people don’t have an AoC yet and until you do combat is a weak spec, second people who played assassination all T15 may not have a good combat weapon so even if they want to switch they are limited. Finally assassination remains an utterly braindead spec with an insultingly low skill cap which makes it a very appealing option. Until now assassination rogues didn’t necessarily have a good reason to go for AoC or a combat weapon because assassination was just as good, after these changes I expect most rogues to move AoC to the top of their list.
The real head scratcher in these changes is the subtlety buff. Subtlety was not hurting on raw single target damage, given the buffs to subtlety on the PTR (approximately 4%) plus today’s buffs subtlety with 4pc and AoC will likely be a very strong single target dps spec, almost certainly stronger than either assassination or combat. Subtlety is still saddled with positional requirements and weaker AoE/multitarget but if the concern is raw dps on uninterrupted single target fights then buffing subtlety by 7-8% seems completely counterproductive. To emphasize, it isn’t that the nerf to assassination is wrong per-say, I’d say its pretty well supported. The problem is nerfing assassination while leaving combat where it is and heavily buffing subtlety just shifts the problem of high single target dps it doesn’t actually fix anything.
All of this doesn’t answer the question most people are really curious about, what spec should I play on Tuesday? As always if you aren’t pushing hardcore progression it doesn’t really matter, all three specs will do competitive dps. For heroic progression my suspicion is if you don’t have a good combat weapon and AoC your best bet is assassination or subtlety. Subtlety’s AoE weakness and positional requirements will make it a weak spec on fights like Nazgrim, Galakras and Spoils so it probably isn’t a good full time spec but for tight single target dps checks it is going to be very strong. If you do have AoC combat you should probably go combat as your primary spec and it is probably worth getting a subtlety spec for big single target fights where it will likely be very strong.