I figure that if you are 21 offense (or more), have 2 dorans blades, are level 9ish, then you can probably lasthit pretty well at this point since your AD advances a lot faster than creep health/armor.

Im not sure how a BFSword is going to help you farm more in that case, vs a finished zerker and zeal. Even without Barrage, the zerker/zeal is going to give you similar DPS to a BFSword, so I'd presume farming is the same speed...and faster with barrage activated.

Assume level 9, 75 base AD, 10 from runes, 5 masteries, 20 from 2 dorans. = lets go with 110 AD. Assume Attack Speed of 0.9 due to AS masteries, base, and several levels worth.

BASE DPS of: 110x.9 = 99 normal, 110+60 x 0.9 = 153 Barrage vs 1k health creep/foe.

You can either have a BFSword (1650g) or a Zerker upgrade and zeal (1700ish).

BFSword boosts DPS to = 140 normal, 193 barrage.

Zerker,Zeal boots to = 146 normal DPS, 218 barrage. (plus the extra movespeed!)

0.9 AS + (0.65speedx45%AS) times 110 AD, (10% chance of extra 110% dmg)

== 1.1925 AS times 110AD x 1.11(crit factor)

Barrage DPS of: 146 DPS + 60onhitx1.1925