I really have to say, after all this time I'm quite disappointed in how the treat the mark of Tzeentch for the demons. That +3 leadership bonus to 'defend against perils of the warp', is a very sad way of saying 'we just didn't want to rewrite the codex entry'.
In the past, demons and others with the mark of Tzeentch had an excessively easy time 'casting' psychic powers. Which made sense, as, not only are demons made of warp magic, their god is pretty much the one who manipulates it best. I seem to recall Tzeentch psychers in one of the codex's not even having to make psychic tests!
When the demons first received their own codex, they used their psychic powers as just 'normal attacks', just shooting them in the shooting phase (or whenever they were supposed to go off), no tests, they just 'did' them cause they're demons, and it's what they do. Though less 'whimsical', this did somewhat make sense considering they're made-of-magic.
Now they've added a special phase into the game especially for psychic powers, and of course, demons should be a part of that. But, the demons who are servants of Tzneetch should be given a bit more of an advantage in this phase, considering their origins, then just 'a slightly better save vs perils'. That +3
LD advantage is there from a time when casting a psychic power meant just making a
LD test to see if it went off. So even when the rule was created, the intent was to make psychic powers 'easier' for Tzeentch demons to cast. Now with Warp charges, that's all changed, and they are stuck generating powers exactly like everyone else.
In my opinion ,there's two ways you could solve this.
The Fluffy option: 
You generate of good amount of warp charges when you play Tzeentch demons, and when I say 'good', I just mean 'good', it's more then most armies, but not enough to be a BIG advantage especially when you consider this their ONLY way of doing any sort of shooting attacks. Tzeentch demons SUCK in melee, so, as in previous editions, they rely on attacking with tons of crazy shooting. Instead of the 'good' amount of dice they generate in the psychic phase, they need to generate an INSANE amount of dice. I recently played a 1000pt Tzeetch demon army, and with 3 x 20 horror squads, 2 x lev 1 heralds, and a lev 2
LoC, I was only pulling down base 13 warp charge dice. This meant my army's shooting was centered around 14-19 dice (depending on my roll that round), Plus, if they rolled well, my opponent could effectively dispel at least one of my attacks with their deny the witch, on top of their armor and invul saves. Realistically, I should be getting something like 30 warp charge dice per turn, more then I could really use effectively, but meaning I could pour as much into getting the spells cast as possible. Even if you take away my +3
LD advantage, you're probably saying "That's too
OP!!!1!!", but hear me out. As mentioned earlier, they are Tzeentch Demons, made of magic, created from the god of magic, yada, yada. However, all this absolute, vomit-overload of psychic powers I'm doing per turn is creating TONS of perils of the warp attacks, which is reducing the numbers of units and killing characters, but that is what these demons would do (according to the fluff). They're completely reckless/fearless/desperate creatures, with not cares for what happens to them. This sort of thing is textbook Chaos. Potential HUGE GAINS balanced by potential HUGE LOSSES. An absolute tidal wave of psychic power, that causes plenty of potential destruction, but also dissipates soon after it hits.
The Sensible option: 
The second fix would be to ditch the +3
LD bonus, and just give Tzeentch demons a +1 to their warp charge activation rolls. This would give them a nice little advantage, but make them fully susceptible to perils, and being more meticulous with how they use their charges (still a 33% chance they fail it, vs a 50% chance). This would also balance them out a bit more when paired with other gods demons in the same army.
Ultimately, the demons just need a new, updated codex for this edition, but I suppose you could say that for any of the armies, it just feels like Tzeentch in particular, got the shaft when 7th ed. hit.
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