Gung Ho lv5 with max attribute: +70 damage
Gung Ho lv15: +100 damage
Concentrate lv5 with max attribute: +111 damage 10 hits
Concentrate lv15: +126 damage 25+ hits (can’t remember exactly but tosbase info is wrong)
So you gain +30 damage for GH and +15 for Conc. Since your main purpose going S3 is for the buffs, there goes 2 ranks down the drain. Since your aim is Corsair, Concentrate applies the damage to each Hexen Dropper and Dust Devil hit, but it only takes 1 hit for each skill off of your 10 available at lv5 meaning you can use all your skills and still have leftover hits from Concentrate so any more than that is not necessary. Concentrate also only applies to the 1st hit of Sacrament and Last Rite so again it’s not really needed for auto attacks. On top of all that, when your buff bar is stacked, popping Concentrate and/or Gung Ho is going to get rid of something much more useful.
Now Barb2:
Warcry: +95 damage for 40 secs with 42 secs downtime.
Frenzy: +150 damage for 40 secs with 50 secs downtime.
Not all that different compared with S3 but with Barb2 you get 3 damage skills and 2 stuns on top of that. Plus Barb might be getting buffed so Barb2 wins hands down. If you really want a build based around buffs go Barb3 where Frenzy and Warcry gets a lot better.
TL;DR
lv5 Gung Ho and lv5 Concentrate is all you’ll ever need.