With persistence and practice, even more should be possible.

I think that it also depends on what you read and in what language. I usually read books in either german or english. German is a bit more complex, but it is the language I am more used to, so I'd say that I am reading both languages somewhat equally. If I were to entirely focus on reading in english, I think that I could further increase my reading speed a bit.

Reading complex scientific papers is of course also more difficult than just a cheap novel. On the other hand, assuming that you really end up being able to read at speeds of 1000 words per minute compared to the average 250, that would mean that you could read the same text four times in the same time.

I believe that this would help reading comprehension much more than reading slow and precise.