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.
It seems pretty destructive as the enterprise had to go to warp to get away from it.
But as for the planet building stuff. No you dont need it so yeah it would be a weaponised version.