OK, so I was right that you were relying on 5 segments. I always thought that the segments of surpirse were capped at 2 when you roll with a 1-2 for surprise. I'll have to go back and reread the DMG.I went over this in another thread, but I'm too lazy to dig it up. If a thief (or the thief's party rolls a 6 on the surprise roll and the bad guys roll a one, there are 5 segments of surprise.
The off-hand weapon was what I missed. I think per the DMG it has to be either a dagger or a hand-axe, so for a thief would have to be a dagger (but could be a hand-axe for the ever popular demi-human fighter/thief!).So a thief with maximum surprise segments would get 5 attacks and 5 attacks with an offhand if fighting with two weapons.
That will certainly give you damage boosts that will keep you ahead of a fighter.