Having been involved in the structural repair of a number of Extra 300's I don't think that the movement of the fuel within the tanks is likely to result in any structural damage, what is likely to be an issue is the lack of a reliable inverted fuel flow from the wing tanks.
I have heard that wing tanks have leaked after rolls with fuel in them. I can't find any official reports.
I think most auxiliary tanks have to transfer to the main tank,
Not on the Extras and Edges. You choose the feed to the engine from either the central aerobatic tank or the two wing tanks together. Its difficult to use all the fuel in the wing tanks as you have to wait until the engine coughs. Any turbulence or banked turns will expose the fuel outlet in one tank and interrupt the feed so the last few litres are only available in still air. This means that you can't easily transition from a ferry flight to an aerobatic one as you could still have fuel in the wings.
My Edge wing tanks are the curved section of the wing in front of the main spar. They are basically the empty space within the wing structure so are not added later. They extend from the wing root to 3/4 of the wingspan. So with a wingspan of around 27 ft they stretch from approx 2ft from the centreline to 10 ft from the centreline and hold about 60 litres of fuel. I think the Extras are similar. The fuel pickup is in the wing root so any bank with low wing fuel means you expose one of the pickups and suck in air. This is why the POH requires the fuel to be on the main acro tank for takeoff and landing.
Roll rate can be 420 degrees per second so the pressure on the outer end from fuel surge during a roll would be considerable I think.