Assassination is the worst specialization a Rogue can play right now. I've tried my absolute best to make Mutilate work, but it's just not possible at a high level (except in certain situations where riding a DK w/ Para might get you a win or two). The nostalgia attached to Mutilate isn't even there anymore. The play style isn't remotely the same anymore.
Why Assassination is LOW tier:
Wound Poison can't be used effectively
Rupture is garbage
Venomous Wounds has a 75% chance to proc
Out of Energy....Out of Energy....Out of Energy....
Envenom hits like a wet noodle
Vendetta is a 2 minute cool-down with a GCD and removed with Vanish, Block and Bubble
If SOME of the above were addressed, I would consider playing Mutilate over Combat and Subtlety.
As for playing Shadow Focus over Subterfuge, you're sacrificing one of your best Offensive/Defensive talents. I would only recommend you pick Shadow Focus in certain situations.
Your openers look fine (openers are situational after all). Keep SnD and Venomous Wounds up, pool your energy for Dispatch/Envenom. Boom roasted.
With this being said, as an assassination advocate, rupture & poisons in Sub does more damage than in assassination, which is really sad. Poisons as assassination over time does more damage, but with how the pace of arenas are right now, you don't have time.
Also, assassination is really energy deprived; with or without Shadow Focus/Subterfuge. One of the only reasons why you can't use assassination at a high end arena. Sure for RBG assassination does great because you're not constantly on top of some one which lets you gather energy.
PTR dmg =/= Live dmg... That's how I see it. You guys are making a huge fuss about what always ends up getting changed in the end.
In the end, either most classes will get a damage buff, or get nerfed and it's always been like that. People QQ about rogues all the fucking time, since vanilla. Nothing has changed; which I find funny because when Ferals were outdamaging almost every class, no one complained about them and they have the ability to stealth too.