This list I made based around Tzeentch is pretty good (in my opinion) at 2K.
2K Tzeentch Marines and Allied DemonsBecause I'm actually considering investing in a Tzeentch army, I feel the need to weigh in on the topic of Tzeentch's effectiveness and psychic powers. I feel like the real potential in terms of powerful armies for Chaos Marines comes from being allied with Chaos Daemons, with the Marines providing the boots on the ground while the Daemons are specialized enough that their side of the army can be tailored to fill the Marine weaknesses.
Nobody has really considered this because Daemons are hostile or risky to most other armies, and are only Battle Brothers to Chaos Marines.
Daemons got an update just prior to the release of 6th edition that didn't get a lot of attention at the time but really makes sense to me now that all the pieces are coming together. Units like Tzeentch Flamers (one of the best units in the game imo) and Tzeentch Screamers work amazingly well in concert with Thousand Sons covering the areas of Horde-control, anti-TEQ, and anti-vehicle strongly.
As far as the Tzeentch Powers go, I think that the fact Aspiring Sorcerers are no longer required to pay for their powers is a great boost, in addition to there being no more Movement Penalties on Slow and Purposeful (The only significant penalty is no Overwatch but that's something not gained rather then lost), no more 'The Sorcerer Commands' BS, as well as no more 'roll more saves' BS when you lose combat thanks to Fearless.
These above boosts considered (not to mention an overall 2 pt reduction in Sorcerer cost) let's get into the powers themselves:
Firestorm of Tzeentch: bad power for a couple of reasons, namely its lack of AP in a predominantly AP3 squad, its random strength that makes it unreliable, all the hoops you have to jump through to make it work (Psychic Test, Deny, Hit, Wound, Save, additional hits). Even if I got Boon I wouldn't switch out for this for all the above reasons.
Boon of Mutation: If the Sorcerer can cast this power on himself (BRB says he can, Codex doesn't specify otherwise) then it is actually a 'good' but not 'great' power. The hits aren't the end of the world (ie they aren't AP1) and in the rare circumstance you fail your save you can always Look Out Sir onto another nearby Tsons model or Character. The benefits on the table are random but a lot of them are pretty good in terms of CC boosting potential. If by chance you turn into a Spawn (you cannot become a Daemon Prince this way) then the Sorcerer is actually BETTER CC support then he would be in his normal form.
Bolt of Tzeentch: This is a good, borderline great power that you have 50% of getting (look at it this way, an Aspiring Sorcerer either gets the power he wants or he plays the lottery with his soul every turn). The key is that it's a beam and not a 1 shot 1 kill it used to be in the last codex (for 30 points nonetheless, ugh). This means you can hit multiple vehicles/models automatically in a single shot with a decent chance of killing them - against MEQ the odds are in your favour to wound for the first 4 models you hit, and you are unlikely to get more then that.
Like I said above, I feel like the real potential comes with
Allied Chaos Daemons to fill in the gaps. When you start considering units in the Chaos Marines book in this context, you can see some definite light spots. Consider the power of a Epidemus/Death Guard list for instance...