I'm skeptical of the claim about subvocalization made here as well, but I'm realistic enough to know that I'm not different enough to fall outside the realms of the study referenced.
 My reading speed in this `mode', so to speak, ranges from 300-750 WPM, depending on how engaged I am in the material. I measure comprehension by testing myself via asking others (with a copy of the material in question) to quiz me on the content after such a reading session.
I think another way to test to see how well someone speed reads is to have them pick out a concept out of non-indexed data. If someone speed reads beyond their ability they will skip over it, if they are not a speed reader it will take some time for them to find it.