Level 18. Killed 'The Beast' on Dominating with my Throwing Knives+Grenades build.
Unfortunately, all the Throwing Knife feats are really just a gimmick for fun. Whenever you throw a knife, you could just as well fire a better weapon, throw another grenade, or just run away for a bit for your cooldown(s) to return.
Caved and took Pack Rathound at level 18 over Pinning, Opportunist, Hypertoxicity and Uncanny Dodge, which were the other candidates at that level. I couldn't take 3 STR carry capacity anymore, not even with the food and Regalia buffs and the lifting belt.
Maybe if I take Pinning, the Throwing Knives will become useful enough that it's worth learning the "extra AP per round" Temporal Manipulation spell. I highly doubt it.