600 point CSM doubles list, advice needed

[Zackdanials]

So i've played 40k for a while now but i'm still getting the hang of sixth edition and have never played chaos space marines before.
Theres a doubles tournament coming up in august (600 points each), i'll be taking CSM's while my friend brings along his trusty Orks and was looking for some advice on the list.
I don't know the full makeup of his list but it involves a warboss, nob squad with big choppa's, a few deff dreads and as many boyz in trukks as he can fit in, plus a smattering of power klaws.

Heres my first attempt:

Thanks for the suggestions guys, here's an updated list:

Sorcerer
Additional mastery level
Sigil of corruption
Spell familiar

20x Chaos Cultists
8x autoguns
Heavy stubber
Champion w/shotgun

10x Chaos space marines
missile launcher
plasma gun
Aspiring champion w/power sword and melta bombs

5x Havoks
3x Autocannons

1x Obliterator
Mark of nurgle


So the idea is his Orks race off and hit as many things as possible while my guys are the backup. The cultists are there to hold down the first objective they can reach (if you're wondering why 8 autoguns it's cause they're the dark vengeance models combined into one squad) while the marines and sorcerer are a jack of all trades unit to take on whatever the Orks miss. The havoks and the oblterator are on anti-armour duty.

So what I think will be a major weakness is flyers. Since 6th edition came out apparently these things are everywhere but i've yet to face one since me and my mates are all too poor for such lovely models, so I'm not sure how to deal with them.
Also I know autocannons arent the best for anti-armour but it's a numbers game, I think six strength eight shots are better than three lascannon shots.
Oh and is the sigil of corruption worth it? Or should I throw those points somewhere else?

Any help would be grand.

[KEO]

Holding the cultists in reserve till the last minute so they can scramble to an objective would be advised. Also arent autocannons strength 7 not 8? Make the cultists bare bones without the autoguns, I know you have the autogun models but save the points cultists arent really good at shooting just tell your opponent beforehand that they are just regular cultists.

Also I would dump the additional mastery level on the sorcerer. You could probably drop the Obliterator as well.

I am not all to familiar with the points saved from dropping these units and reducing wargear but if it does give you enough points get a quad gun and an aegis line.

[Zackdanials]

My bad, they are strength seven, but hopefully no ones bringing an AV14 tank to a six hundred point game. But isn't that all the more reason to keep the obliterator in? His las cannon/multi-melta/twin linked meltagun would be even more useful, although I'm very wary if deep striking him. I have a habit of scattering them into rock, enemy units and the next table over.

I'd quite happily just have barebones cultists, but how strict will they be on WYSIWYG at a GW tournament? I have a sneaking suspicion they wouldn't let me get away with that...