I was a bit surprised to hear from this P-51 pilot (9:33 into the video) that the Mustang requires 9000 feet to recover from a spin. This seems high to me. The Mustang POH does not confirm this, but given the indicated recovery procedure, it seems possible:
b. RECOVERY. -- Recovery procedure is the same in both a left and right spin. Upon application of opposite rudder, the nose drops slightly and the spin speeds up rapidly for 1 1/4 turns, after which the spin stops. Rudder force is light at first, becomes very heavy for a period of about one second at the first half turn after starting recovery, then drops to zero as the spin stops. Recovery is effected in the normal manner, that is, by applying full opposite rudder followed by movement of the stick to neutral.
I've researched altitude loss in more modern GA single engine planes, and <= 1K per turn seems common. See this discussion about altitude loss in a 77 Mooney M20J.
I've also researched altitude loss in the Mustang a bit further, but have been unable to find confirmation of this. Now I don't imagine we have a plethora of Mustang pilots actively participating in this forum, so I'll temper my question to be, does 9000 feet seem reasonable? If so, why?
Note: It is not my intention to call into question what the pilot in the video is saying. He does admit he hasn't actually spun the mustang, implying he either read or heard this elsewhere.
Update, July 21st 2017
I came across this video, a VERY good 1944 briefing likely filmed for pilots who are being introduced to the P-51 (Have to admit, I never knew Lee J. Cobb was balding!) . The pilot demonstrates both left and right spins starting at 20:58. It is mentioned that it takes 3 turns to recover, but unfortunately, it is never mentioned how much altitude is lost. However, I suppose it is possible that it could be estimated by looking at the plane against the background by someone far better at this type of math than me.
I've also reached out to the Flight Chops guy to see if he or the pilot want to add their two cents to the discussion.