Does it really take 9000 feet to recover from a spin in a P-51 Mustang?
9000 feet is not likely if the pilot applied recovery techniques in a timely manner. 

US Army Air Force spin tests found here resulted in an average of 3000-6000 ft alt loss. There are several memos on it all resulting in similar figures. 

This one dated 6/26/1943 shows 5500 feet for P-51A



This one from 4/30/1944 shows 3000-6500 ft for P-51B




The longest drop noted was on this memo from 7/10/1943 on the P-51A which indicated a single occurrence where the pilot modified his recovery technique resulting in a drop of 7500 ft.