Its an interesting list.
I would do the following
For the Havocs re-arm as follows
-- Havocs (5)
- 2 Heavy Bolters
- 2 Autocannons
-- Havocs (5)
- 2 Missile Launchers
- 2 Lascannons
Its more synergetic that way round.
Boost the
CSM squad to 10 and take a second Plasma Gun, drop the plasma pistol and powerfist. The powerfist is an anti tank weapon which goes against having plasma guns.
I'd drop the cultists and Flamers and try and fit in a small
CSM squad with a melta gun.
Only other thing I see is your 10 man
CSM squad running away as they wont be fearless, you would need a Lord to give them that if your keeping the banner (which is great). If you do take a Lord pick a Slaanesh one and give him twin lightning claws
Thats my thoughts.
Edit: For the Slaanesh lord you could use the chosen model with two lightning claws