Let's face it.... being ale to have 5 crew that give you +10 defense or acc would be broken.... I could MAYBE see them doing something like the Borg Boffs with a Joined Trill though.
I agree that 5 boffs with +10 def or acc would be an issue and not at all what id be looking for. At a minimum KDF needs a teamwork boff. But additionally what id like to see is tac boffs with a passive space trait that reduces the drain by energy weapons. A engineering boff that buffs engineering team by adding a +5 all system power applied to target ET is applied to.
Something as simple as a buff that affects the respective team would be kinda cool. But something that addresses a boffs specailty training.
I think of something like "Piloting" trait that increases mobility and evasive maneuvers effectiveness.