In my experience, subvocalization doesn't become a barrier until you hit maybe 900-1000 wpm. I still subvocalize, and I read at about 800 wpm with appropriate software and 500 wpm on dead trees, so it's definitely achievable. Over the span of several weeks, I increased my speed from ~250 wpm by spending 30 minutes a day practicing the techniques from Matt Fallshaw's presentation at the Effective Altruism Summit. Unfortunately, my notes are about 3000 miles away, right now.