The most I've pulled IRL is 4g (obviously vertical), and it just squashes you into your seat, you do NOT get 'thrown around'. Generally, when you're going to fly an aircraft that is capable of these kind of g-loads, you get strapped in
really tight. Very rapid roll rates (ie 300 dps+) do tend to slam your head to one side, but if you're the guy flying you're going to know it's coming and try to counter
(When you're not the guy flying, it sucks) I've stalled light aircraft literally hundreds (if not thousands) of times, and that doesn't even come close to 'shaking' your vision in such a way that it becomes noticeable. Sure, sure, Cherokee's, C172's etc aren't Spitfires, but hey. The worst bouncing I've ever experienced in aircraft is during (very) rough turbulence, and rough runways. So, I wouldn't raise the headshake levels much, if at all. One funny point I've noticed, not sure if others agree, is that with Track IR, I end up having to reset it every so often because my sight line has ended up low - almost as if the pilot has permanently 'shrunk' from pulling g's!
I can't say I've fired guns in aircraft (from ships, but that doesn't count I guess - I've stood right next to 20mm cannon firing from a patrol craft, and that certainly didn't bounce me around), but I work with several guys who have, obviously jets though (Mirage F1 & III, 2X30mm cannon mounted right under the pilots butt), and while they say you can certainly feel it, it doesn't bounce your vision around. And those are larger calibre, higher ROF guns, mounted closer to the pilot (on an admittedly heavier airframe) - I therefore also firmly believe the 109 cannon headshake levels are ridiculously high. I feel like on pulling the trigger, Mike Tyson has suddenly decided to do that funny speed boxing bag exercise with my head - totally kills the immersion.