Interesting thoughts, I do believe that the numerous appearances of The Borg in Voyager did end up watering them down.
However, I feel that Sisko's hostility originally directed towards Picard was simply because he had been Locutus, and facing "normal" Borg may have still had an effect on him psychologically.
One thought, as Sisko is a trained Starfleet officer, obviously fully briefed on the Borg considering he was working on the design of the Defiant, he still allowed his feelings to get the better of him. I wonder if Picard had recieved hate-mail, threats, or even possibly experienced assaults from the relatives of those who'd died at Wolf 359?
Of course, it would have been great had they been able to portray Sisko in command of the Defiant in First Contact, but hard to accomplish with him being the main character of DS9.
I am glad that earlier scripts for First Contact showing Sisko sending Worf in the Defiant to defend Earth were not included, as IMHO Sisko would have insisted on commanding the ship himself.