What Psychic Power will your Great Unclean one have? I'm inclined to go with Gift of Chaos (he should be possessing either close to or in combat, so will spend most of the game in assualt, where GoC is best), but Wind might be ok depending on the armies you expect to fight most often.
You're severely lacking in long range firepower. Your Defiler is your only decent source of tankhunting, and it's prone to getting blown up first or second turn. Armies with lots of armour will likely chew you up pretty fast.
I'm not a huge supporter of the Defiler. For 145 you can get either a Predator (Autocannon, Las Sponsons, Daemonic Possession) or Dreadnought (TL Lascannons, Daemonic Possession) with similar tankhunting power (better in the case of the Pred), and the Battle Cannon isn't all that it's made out to be. What you need is more long range, accurate tank-hunting, and the Defiler is just too much and too versatile for what you need. The Battle Cannon is fairly unnecessary, as that's what your DG are for (don't be afraid of 3+ save armies; you've got the same, but you're tougher, and you've got extra plasma if you really need it). Replacing the Defiler with a Predator would give you more points to throw around too, even if you wanted to go a little overkill and take either TL Lascannons instead of the Autocannon, or Mutated Hull, or take more Aspiring Champions (which I recommend, definitely with Daemonic Strength).
The lack of ACs is the main problem I see right now, as you're fairly lacking in CC ability (those invulnerable ACs with 3-4 S5 Power Weapon attacks every turn are what win you the assualt phase, not all of those extra plaguemarines).
As for a rating, maybe 3/5, since you're close, but there's still a fair bit to fix up.