Gaiden said:
Let's presume you start with a Str of 14 (12 after penalty). Your average damage against an AC 20 opponent (reasonable average for opponents at 6th level) will be 8.365 (assuming weapons of at least masterwork quality). Not to be harsh, but this is utterly pitiful. Moreover, you won't have the hit points to stay in melee and even if you've flanked you still only get one attack.
For a knife thrower to be viable you MUST increase your number of attacks.
Ok, lets compare some other builds then. Same race, same stats.
Rapid Shot Rogue. Rog4/Ftr2
Replace Expertise with Improved Init and Improved Feint with Rapid Shot. In the first round you will murder your flat footed opponents but what about round 2+?
Attack at +10/+10 for 1d4 + 1 each. Avg 6 dmg/rnd (Advantage: can attack from beyond 30 ft but this guy still doesn't have Far Shot yet)
What about a straight fighter then? Ftr6
Replace Expertise with Far Shot and Improved Feint with Rapid Shot. Pick up Wep Focus and Wep Spec.
Attack at +12/+12/+7 for 1d4 + 3 each. Avg 15 dmg/rnd
Ok so the fighter kicks both their butts and has more HP to boot. But now we come to the point that the original poster wanted an "acrobat knife thrower" and this fighter is decidedly not that. Like I said before, the Rog gets all those skills to play with which is what will make this character fun and memorable, the better than average damage output is just a bonus.
Plus, the cool thing about Sneak Attack is that it scales with level. Lets compare our two rogues at level 11 (when the Ftr is also gaining another attack).
Sneak Attack Rog9/Ftr2
Attack at +15 for 1d4 + 5d6 + 1. Avg 18 dmg/rnd
Rapid Shot Rog9/Ftr2
Attack at +13/+13/+8 for 1d4 + 1 each. Avg 9 dmg/rnd
Plus the Sneak attacker has the advantage of attacking once at his highest BAB, it is wishfull thinking to assume those itterative attacks are always going to hit.
Honestly, none of these builds are really twinked out compared to what else you could do with those levels and feats. But they sure do have style, and sometimes that is all that really matters right?