For doff missions, look at the various million thread Heretic has been posting in :p
For the Kar'fi, it is a unique ship, different to that of the Vo'Quv, starting with far less hull and superior turn rate (not to mention the Boff layout). Since it has less hull, you don't want to tank with it, with superior turn rate, I recommend at least one dual heavy cannon on the front of your ship and the boff layout should be something tactically based so you can dish out damage.
Using reverse and evasive manouvers is a good tactic to flip your ship around quickly and brining your cannons to bear. I've seen build with weapons of that of an escort (all dual cannons and turrets) work perfectly because they also use debuffs like Tykens Rift 3 and Scramble Sensors.
A tactical layout is your first choice I'd say and I suggest you go with it, since it is more tactically based than the Vo'Quv. Equipping Dual Heavies, Turrets and Debuffing powers will gain you a significant advantage. But all of this relies on your own ability to get your ship turning and flipping quickly so your opponent is in the sights of your cannons. With the right combo of powers and maneuvers, the Kar'fi will be a fearsome adversary.