I'm one of those people who isn't too sure he buys the whole thesis, but this may just be because I really don't "get" Faustian culture. (But, then, what do I get? Got me.)

But, as a data point in Spengler's favor (and to a central point in your essay), Gilbert Murray also talks about how the Neoplatonists were cut from a different cloth than earlier Hellenistic thought generally. It's also interesting that some of the central figures of that movement, Porphyry and Iamblichus, were Syrian rather than Greek.