The g-force experienced by the 'stunt' pilots are very considerable very high! They dont need to wear a g-suit because they only experience this sensation for a short period of time.
Remember all their aircraft ie the various Extra 300's etc are piston aircraft with a whooping great big engine and prop that produces an immense amount of torque. This is why many of these aircraft have a thrust to weight ratio sometimes exceeding 1:1 ie 2:1 for example. Also remember that the aircraft have near to no weight in them. The only real heavy part of the a/c is its 'scaffolding' which is stressed to high heaven and capable of withstanding the high 'g'. Although the 'g' experienced is high, it is not sustained so the blood loss is very quick, however the manouvre is split second in some cases and so the body is quick to recover.
Unlike fighter pilots in jets they need a g-suit because the magority of the time they are under sustained 'g' ie for a lengthy period of time in some cases for a few minutes or so. Therefore, although they may not be pulling 7,8 or 9g they will pull 3, 4 or 5g for a longer period of time. It is because of this that they need the g-suit to prevent blood loss from the brain (cranium) and torso pooling to the lower limbs such as the feet. So, the g-suit is set to approximately 2 - 3g and inflate like when you have your bloodpressure checked by the doctors, it stops the blood flow from running and in doing so it keeps the blood at where it is, esuring that the vital organs such as the brain and heart have the blood when its needed.
The reason you get black out, is because the blood has been 'drained' from your upper body starting with your brain, making you feel woosy and lightheaded, then it 'drains' from your eyes, now you start to lose your poriferal - spelling? vision, which now encroaches on your forward vision getting smaller and smaller. You are now experiencing 'tunnel vision', however you cant see the light at the end of the tunnel and so it closes in on you and you pass or 'blackout'.