Is your question if the motor and prop combination in the ARF kit will be sluggish with the bulk of FPV hardware? Or is it that the Naze32 will be sluggish with a bloated 250 size quad?
The motor and prop combination in the ARF kit will have plenty of power for lifting a FatShark setup. A small video camera and the 250mw transmitter adds less than 100 grams of bulk. If i understand correctly the ARF kit should be in the realm of 375 grams of all up weight. As long as you keep the all up under 500 grams, there will be plenty of power for zipping around while flying FPV. It wont however do well to lift a GoPro and FPV hardware, it will be sluggish and flight times will be minimal. Once you've gotten a dozen hours of flight time under your belt and want just a little more power, going to 6x3x2 props will give just enough more power to max the motors in current and give a bit more pep.
The Naze32, or any MultiWii based controller, is not for the first time builder. Once you've built a rig on a KK2 and tinkered with it enough to understand all of the settings, then going to a Naze32 or CC3D will be an easier learning curve. That learning curve includes how to configure advanced flight features (if the hardware supports it), tuning gains to run small or large span rigs smoothly, and how to dial in just how aggressive the board will allow you to fly. I found the Acro Naze32 to be aggressive enough "out of the box" for a fun FPV craft for my Bat Bone with 8.5" booms; but it was too docile to fly line of sight and swoop around like i tend to do. I've dialed up the aggressiveness so it will fling around with more authority, which would be too much to handle while flying FPV. So the Naze32 can be tuned up or down to suit your taste, you just have to have enough experience under your belt to understand how you want it to feel for how you intend to fly it.
