A lot of the sound you are hearing with that P-51 is the prop and the fact that he is keeping the throttle full open on most of those passes.
As far as room needed for 1/3 scale, it depends somewhat on the plane and how the pilot flies. The flying field behind cut4fun's house is nowhere as big as the runway area at the local airport on the west side of the town where I live but at cut4fun's house there is no airport with full size planes flying close by and the closest house is 1/2 mile away. The runway at the model flying field is only 300' long but with over runs and clear back of the trees. It used to been a skeet shooting area and was timbered out around 10 years ago.