Very interesting discussion and it was great that you got Harry from VPI to discuss the different length arms. He set up and described a legitimate test. Here is a quote from him:
Truth be told I am the only one who has heard 9", 10.5", and 12" Fat Boys mounted on the same Direct Drive Avenger using 3 Ortofon A-95's and can state without a doubt they sound different and I prefer the master tape smoothness of the 12". Besides, I have the same Ampex ATR-102 that Chad uses to make the records, totally rebuilt by ATR Services. I have master tape copies of the music and the 12" sounds more like the master than the 9" or the 10.5".
So, I guess VPI is one of the few manufacturers who did not tell MF privately that they only provide 12" arms to appease customers. Thanks Fsonicsmith for sharing a link to this discussion.