Hmm... I can obviously not claim you are wrong. But I do find the difference between internally hearing something and "subvocalization" tough to distinguish.
And, my entire point was that reading is different to different people. So... it may well be that many folks can not speed read. Might even be the case that most speed readers are not as good as they believe. Still, to see the numbers in this, 600wpm with 75% retention sounds ridiculously fast and not too shabby on the retention. So... yeah?