Why not build a few of these. Sure they could destroy a Cube.
I think the Klingons and Romulans would understand if its for borg bashing.
I don't believe that the Genesis weapon/torpedo is such a kill-all weapon.
I would say that if detonated it transforms matter into a M-class planet with life. But, I don't believe that it would work against a shielded enemy. Maybe first get the enemy's shields down and then Genesis eliminates them.
But even then, a proto-Genesis would me more effective, after all we don't need planets (and if they are created in the wrong place all life on them would still die out). Just destruction would be enough, so a singularity/black hole would be better - to destroy ought to be enough, no need for creation.