I did not invent the T-6 connection, nor do I claim any great experience in the type, nor do I suggest it's the only way to prepare oneself for the Mustang.
I'm only pointing out the historical fact that a great majority of military pilots came to fly the P-51 via the T-6 (AT-6) route, and the ones I've talked to had high regard for the preparation it gave them. Therefore - based on this proven sequence of instruction - I conclude that this pilot probably would have benefitted from a similar syllabus.