Should i dump points in suppression somewhere ?
Suppresion is a touchy one, different people have different opinions on it, and it really depends on what you do most of the time. I don't have a single point there, and will only be putting one there at 70 as i can't really find a better place for my last point. Some people like to have 2-3 points there, but most agree 5 points is too much. In the end it's up to you. If your getting a lot of resists, you may want to drop a couple points into it.
How do you rate improved drain soul ? Should i put 2, or simply ignore it when i respec?
For Drain Hunters, this talent is part of thier bread and butter. 10% less threat is great. Drain Tankers on the other hand want enemies bashing on them, which is still easy to do even with this talent, and getting some mana back at the end of the fight is nice. Even if it doesn't always heppen. It also helps in instances and later in raids, as a squishy getting less threat is always a good thing. If it were me, I'd leave it a 1 for now, and later when you get Dark Pact, re-spec and move the point from Life Tap there. Again it's entirely up to you and depends on how you play.
Which skill should i try to max at this point ? CoA , Fei concentration, or amp curse? My guess is that it doesn't really matter because all of them are still useful, but i am still convinced one of them could probably help me more than the others short-term.
It all depends on your playstyle. If you are planning on being a Drain Tanker, max Fel Concentration ASAP. Without it, Drains get interupted and your not really a Drain Tank. I wouldn't worry with Imp CoA, until later in the game when your fighting raid bosses, CoA never really has a chance to run all the way through. It's damage is back loaded, so is the bonus, so unless the last few ticks go off, most of the bonus from the talent is never seen. IMO, it is not a good levelling talent. Amplify Curse is great, but for me, only a step to get the CoEx. Again, Amplify Curse does not seem to me, to be a good levelling talent. CoEx however, is great, especially Amplified, when you have to kite that elite, or get away from that player.
Which tree to compliment is again, entirely dependant on your playstyle. Most Affliction locks grab a little from both Demo and Destro though. Demonic Embrace is practically a must. I like Imp. Imp since I always have him out as a mana battery, and the extra Sta from Blood Pact is great. Some would rather take Imp HS though, it's ultimately up to you.
One thing most Affl locks tend to agree on though, is Shadowburn is great. Imp SB, Bane and Shadowburn are a great investment in the Destro tree. In instances, between DoT cycles, you'll probably be spammin shadowbolts. This helps that, and Shadowburn gives us some nice burst, great as a finishing move.
I have seem Afflic locks not put any in Destro, and go pure Afflic/Demo. I was actually thinking of a Shadow Mastery/Soul Link build myself at one point. I'm sure it would work, if it fits your playstyle. Didn't want to give up Dark Pact though.