Sir,
i dont want to disappoint you, but in a stallfight, the G14 has no chance agains the spit16, that spitty can do everything better than the Gustav. Against an equal pilot, you will lose every single time.
What you can try?
I like the fast snap-rolls, repeat them many times, burn as much speed as i can, open the flaps, kick the rudder, chop throttle, and you may have a chance til the spitty realizes that you got behind him and starts a sustained turn or finally can open its flaps. You have two very small speed-windows when the planes are almost equal: 165-180mph (second flapnotch) and under 100mph where the nose-heavy 109 can gain advantage in the vertical hammerhead-style reversals.
I dont like to turn with the spitfires if i have too much fuel. In the G-6 i avoid the turnfights with them when i have more than 50% fuel. The G-14 isnt as much heavyer than the G-6, even with the tater gun, what gives you an awsome firepower for the snapshots. I never fly with the gondolas on, it may be hard to aim the tater, but once you get used to it, it makes you almost unstoppable.
There are virtuoses, like Krupnski, who mastered the tailslide tactics. It can be useful if you exactly know how the plane will turn to the control inputs, but can turn you into an easy, big target right in front of the opponents guns. Ask a muppet, they will show you how to reverse in the top of the scissors, basically 70% throttle, pull the stick full back n left plus full right rudder at very low speeds and the plane will turn instantly nose-down.