Does it happen a lot that you don't have enough power dice for casting with 2 x 5 Warlocks and a sorceres or supreme sorceres?

Quite often depends on your luck.
Personally I tend to run a max of 6 levels of magic to mitigate the problem. Obviously the more casters you have the more likely you are to run out.

You should start the magic phase with a plan of which spells you want to cast and ensure you have the dice. Any you manage on top is a bonus.

Ok. And every warlock unit can cast Doombolt in a turn right? Normally a spell can't be casted twice, but in this case it can?

Funnily enough, this is something I've been coming to realise in recent games. Why am I bothering with a level 4, when a bubble Soulblight or a Doombolt can usually achieve whatever I'd want a level 4 to do, without the penalty of miscasting, and with having two of each to force through? Most of the time it's only when I get big magic phases that I start to think about what the Level 4 can do, or otherwise if she has a key spell that can help me win the game.

Otherwise, she's there for the boost to dispelling. And that's a lot of points for +2, but I'd feel pretty vulnerable without it.

Perhaps this is an issue with magic lores - I often take Metal or Dark, which are both aggressive like Soulblight and Doombolt, so there's a lot of overlap. If I was running Light, Life, Heavens or Beasts I might feel differently about it.

It is important to be able to shut down opponents magic though. When your top mage is lv2 and his is lv 4, every dispell attempt is at a disadvantage. This adds up over time.

A big advantage of a lvl4 is flexibility. Soulblight might be useless if there is no combat and nothing important to shoot at - which happens quite often in the first turn as my opponents tend to hide what they can in deployment, trading a few inches of advance for making my first turn useless. Similarly, doombolt is not that good against 1+ armour for example. Certainly not as good as searing doom.

In addition, if you have two units of warlocks and no additional casters you might want to keep them safe and not use them to hunt terrorgheists and such. This might mean that you are actually paying 250 points or more for something that does nothing but casts during the game which is a price comparable to the price of a lvl4. I suppose a good enough player can avoid this trap, though.

Last, because the casting values are relatively high you might feel forced to just grab all dice you can and throw then on a single spell. While this most certainly works you can often slip through two spells or even more if you have many spells at your disposal.

Therefore, in my playstyle I consider a good balance to have a lvl4 + warlocks or lvl4 + a support caster. And I could imagine a bigger investment to work as well - for example, an extremely shooty army might benefit from lvl4 metal + lvl4 shadow, for example, which is what wood elves are often doing these days.

I understand now. And good thinking, Marchosias.

Nice summation Marchosias.

To resolve your problem you have 2 options
1. Roll better
2. Take less magic.

I love combining two lock units and a Supreme Sorc. I like the bonus on the casting of the Sorceress. For Doombolt, that's +5 instead for the Sorceress instead of the Lock's +2 which is considerable and saves a single power dice on the spell. The third channeling helps out which, combined with the Sorc's bonus, could make a spell of difference.
Lastly, I like the Supreme Sorceress simply because she offers more variety in the spells. That flexibility makes the whole magic phase more dreadful for my opponents

well I run a L4 on light a l2 on heavens and I have 1 unit of warlocks as they are too good not to include.
But I must admit I usually use warlocks at turn one to cast a dooombolt then possibly later game a soul blight to drain opponents dice.

I've been playing most of my games recently with ET magic, with a Level 4 on high, warlocks and sisters, meaning I have access to 5 lores and 36 spells, so never have enough to cast what I want.

I think the thing with having a few different casters is the options you have available? Like does your opponent want to stop the soulblight, which could damage the unit in combat and shooting, or the doombolt they know is round the corner, that kinda thing. With ET magic, opens it up a lot more, means your opponent has to be more selective about what they try to stop.