its not as fun
It will be less fun when your aim is very restricted. And, a mounted gun, if collision isn't done right, will end up shooting the plane that it's mounted on.
It's also possible to mount other turrets such as the pirate cannon or tank turret onto vehicles, but it requires a console command.
So what I was thinking, we could create events to do this, and whenever the plane spawn, a tank turret and a mounted MG could be mounted onto the plane. Bushi's sentry mod could do this as well if you want to have automated guns. Bushido's sentry guns can mount to vehicles if you use a wand or something to push them into it.