Warpflame+covens=profit?

So a thread on Dakka made me think recently. How profitable would it be to run a Haemonculus Covens force alongside Tzeentchian daemons? Specifically, ones spamming as many of the warpflame powers as possible.

The covens units could deepstrike in, in order to tie the enemy up in CC, leaving the psykers free to tear things up at range. Plus, the two powers with Blast can be positioned over an enemy unit, in such a way that they still clip a covens unit, potentially raising their FNP.

What are the rules for this warp flames? And who can use it?

It's a special rule on all Tzeentch witchfires. Any unit that takes a wound from it makes a toughness test. If they fail they take d3 more wounds, if they make it they either gain FNP (6+) or add +1 to FNP rolls if they already had it. (Meaning you can potentially get up to a 1+ FNP)