I suppose it doesn't increase the knockback effect of your Fus, does it?
Not really sure on that, but the perk you can get from apocrypha seems to.
By how much? Does it allow you to rely primarily on them?
Not really. It adds another 25 percent reduction, so if you wear the talos amulet and have talos's blessing, you can have a total of 75 percent reduction. Its a lot, but the power lasts for like 4 minutes I believe, and it has a once a day time limit after using the shout. Understandable, since it does tend to make some fights easy. Also, when you're low on health, it summons a badass dovahkiin spirit to fight for you. the first word does this after 10 percent health I believe, and all three summons it after 50. It also adds like 100 armor, and increases melee damage.