Agreed. The Mark of Khorne adds half again the points but only adds half again the attacks, at best, for one round. You're actually making yourself worse in combat relative to just adding more dudes.
I'll be honest, I think most of the marks are pretty useless on Cultists. The only one I'd consider is Tzeench IF you have some way to reliably give the unit an invulnerable save (a skyshield landing pad, or a coterie of divination psykers), but even that raises the 'so what' question because they're not going to achieve much even if they survive.
I realise in this case everything has the mark of Khorne "just because" and that it's a khorne army.
So....marine-wise, I'm kind of shooting blind here, because I've read the experimental rules but my copy of
IA13 arrives tomorrow-ish.
Assuming they've not changed much, though, there is a flaw in your plan - the Drop Pod Assault rule reads
"In the case of the Kharybdis, unlike other Drop Pods it is not immobile, however, after it has landed it is treated as a Flyer with Hover mode (which starts off as hovering after it has arrived via Deep Strike)."
Which means it lands - as in actually lands lands - the turn it arrives from reserve. You can stay inside it (indeed that's probably a good plan - you can't charge so might as well stay inside and heat blast) but you are 'on the ground' and the assault claw can be shot and assaulted normally.
You can't go far wrong with a tide of cultists with a helbrute and daemons following them up.
The bezerkers themselves are a lethal unit. The one reservation I have is the champion:
~ Power axe and Lightning Claw is an odd combination because it costs you an attack - one has the Specialist Weapon rule and the other doesn't.
~ The combination is presumably there for flexibility but the champion doesn't really need it.
~ You have Kharn the Betrayer in the unit. He is a holywhatthebejeezuschainaxefacebreakerdeath monster in challenges thanks to his weapons and abilities.
~ Excess wounds in challenges now go onto the unit, so you're not getting 'tied up' by accepting challenges, you're just killing the important models first.
~ If Kharn doesn't want to accept the challenge, it'll be because the challenger is something that no aspiring champion is going to stand a realistic chance of beating in combat, no matter what he's armed with.
~ You might as well stick with the Power Axe - it's the most cost efficient squad-butcher weapon available to chaos marines, and with 18 regular bezerkers to get killed first, you shouldn't be especially concerned about it being unwieldy.
~ I might take meltabombs instead. Extra armourbane attacks never hurt.