As an anecdotal counterpoint, I tend to read fiction very quickly compared to non-fiction, and enjoy it / retain it well enough for me. When I try to read most non-fiction that quickly, I usually don't get much out of it.
Same thing here. When reading fiction (and some types of non-fiction) I can do away with my internal monologue and absorb the text directly, with full or near-full comprehension.  With textbooks etc. I need to actually subvocalize the words for full comprehension.
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.
Strangely enough, I find that retention is always better (for me atleast) in casual reading of novels etc; I'm ok there with ~400-500 wpm. I do skim through reports actually, well mostly that happens when you approximately know the content so there just may be higher speeds may be achieved as you know what to expect. I think for me it is mostly prior knowledge that maybe makes a difference of 100-200 wpm
Me too. What varies my speed in fiction reading (beyond plain badly written fiction) is the level of detail the story forces my mind to create to envision it. In a fiction book I really enjoy I'll spend more time 'dreaming' it then reading.
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.