Those new Naish Guns look like "Super-Guns", in terms of wide point in forward, and a quite pronounced "Pin Tail". A real Pin Tail, not just a Rounded Pin, Semi-Gun style.
Another thing that is important to look at, is the amount of tail rocker. And the amount of final tail rocker through the final 1 to 1.5 feet of the board.
Tail - Final tail rocker adds to your maneuverability (tough word to spell!!

)... But minimal final tail rocker should deliver you more speed, but the board will likely feel much "stiffer" through turns, including off the bottom. Especially if paired up with bigger fins. Pin Tails also tend to have this feel/effect.
Fins on Guns... for small waves, bigger fins can usually give you more drive. But if you are on a big wave, the actually drop should give you all the speed you could possibly want, so the big fins are going to stiffen things up a little when you turn... I would suggest slightly smaller side fins in big waves (compared to your regular side fins). The side thruster fins should also be set-up with less toe-in, but that's usually the shapers responsibility.
Note - Toe-in seems to have been eliminated on the new JP SUP's.
Disclosure - my preferred big wave weapon is a 9'6 McTavish Longboard Gun.
Also note that guys were successfully riding big waves on Single Fins for many decades, before the advent multi-fins. If you are even in Torquay, take particular note of the Ian Kanga Cairns Waimea Gun that they have in the museum.