This is exactly what my Magik is wearing and how I've got her points applied right now; I am fairly low on Infinity but haven't really run into any issues with this build yet.

Obviously it's not 100% ideal, but I'm not gonna throw up a Cosmic GOK in this build until I get one to drop; these are simply the best pieces of gear I own for this build.

I like the Cosmic slot 3 over her Mantle of Limbo's Queen because it happened to drop and the stats just feel better; the CDR on the cosmic is better than the BDR on the unique, the cosmic loses out on +2 core stats (equal to 320 DR) ~480 Mental DR and ~500 Summon DR and 10% attack speed, but it has a proc for 8% attack speed and 800 DR which makes that loss a lot better, and it has better BHR and HP plus an invuln proc.  And the -2 second cooldown on Otherworldly Nova is irrelevant when you're trying to cast it at the same time as Back From Limbo to apply the maximum power buff anyway.  

Maybe I'm wrong and the math works out to be better for the unique but it sure seems close to me from what I've played.

Out of combat:                                                                                      In combat:

 plus insignia proc

Full stats pane:

Critical Hit Chance: 45%

Brutal Strike Chance: 41.9%

Critical Hit Damage: 332%

Brutal Strike Damage: 550%