I played against this very band the other night at Vassal. And in the end managed to win quite comfortably. I figured I ought to take my own advice and play a band I had proposed in another thread (w/minor fodder modifications) and so I played:

41 Dark Naga
117 Bluespawn Stormlizard x3
26 Destrachan
12 Greenspawn sneak x2
3 Warrior Skeleton

Opponent won map init and chose Hellspike Prison. At first it seemed a poor matchup for my band. Marilith has immune electricity and his Redspawn Firebelchers could get immune electricity by being adjacent to my Stormlizards. Meanwhile I would only get resist fire 5 for my Stormlizards by being adjacent to his belchers. Anyhow I thought I'd better use the electricity bolts of the Stormlizards and the Dark Naga against his belchers early on before he could base my units and gain immune electricity. To these five lines my opponent does not fail a single save. Uh-oh... Still I manage to pop one or two Orc Warriors of his and get ahead on activations. Due to the Greenspawn scouts I also am ahead on points, forcing him to seek to engage me. Basically for the rest of the game I stay out of the Marilith's reach allowing it only the occasional aoo, preferring instead to engage his other pieces. A few init's won does help with this. In the end the Marilith is his only piece left while I only lose one Stormlizard and some fodder. So.. poor matchup or not, it was not poor enough to lose the game.

I think, not only may a +5 damage bonus be the best cfx for the Marilith, I'd doubt whether it's worth it without such a cfx. The Marilith may on its own potentially cause 60 damage @ +15 for 73 pts. The Eye of Gruumsh can potentially cause 50 damage @ +15 for 44 pts. While there are other considerations, in terms of pure damage output the EoG does seem superior to the Marilith. If you go with the Marilith it should then be for another reason than it's damage output.

A fairly straightforward Marilith band might be as follows:

55 Ryld Argith
73 Marilith
26 Doomguard
40 4x Quaggoth slave
6 Fodder

Basically it aims to ensure that the Marilith can do what it maybe does best, engage a select target, win initiative and unleash a full turn's worth of devastating attacks. Quaggoth's attacking at +10 for 20 pts of damage w/cfx ain't no pushover either and aoo's from these might prove rewarding, should the opponent elect to risk them. And one should presumably try to place them so as to make it impossible for an opponent to run from the Marilith without suffering aoo's from them as well.