Don't get me wrong I really dig the Seer Council Formation. I think when you do it, you need to do it property. In my view that means getting all the equipment needed to make sure you don't suffer things like, overwatch, running away from combat etc.
As for the Wraithknight being better than 5 warlocks... well. I really don't think that's correct because:
Wraithknight has hammer of wrath (nice against armour or a cheeky instant death)
he has D-weapon attacks with 1 mastercrafted.
he has 1-3 stomps ( I recently played a game where I took out 800 points in stomps rolling double 6's)
he is toughness high so, immune to bolter fire and a lot of melee attacks.
He causes fear, can blind people and is just.. like so much better than what you could ever do with 5 psychic dice and access to runes of battle XD
Just to clarify I am not saying a wraithknight is better than a seer council, im saying its better than 5 warlocks.
Even then, if you have a fully decked out Seer council, it should cost you around 800 ish points. You can take 2 and 2/3 wraithknights for that and I would rather have 3 wraithknights XD but again I like to play competitively so in all honestly whatever you think and want to play is the right choice. It's all about fun at the end of the day