Typing at the maximum possible speed, to send qrq cw on a morse code keyboard, it has been my experience that listening to the sidetone has significantly lowered my maximum qrq cw typing speed.
W3njz told me a couple of years ago to turn off the sidetone in order to optimize my ability to send CW on a keyboard at my highest possible speed. It was very awkward at first, but after about 3 weeks, I finally started to feel comfortable not hearing my sidetone and noticed that my typing speed did increase significantly over those sidetone-LESS 3 weeks in comparison to the improvements I had made over the last year of practicing speed typing with the sidetone. I focused my attention for accuracy to visual feedback instead of auditory and looked at the screen for error correction. The more text that I was able to type ahead of the buffer, the more time I had to correct spelling mistakes. VFEC, Visual feedback error correction, is performed in REAL TIME and gives the best chance for top speeds. When you are sending cw at your top speed, and just barely staying ahead of the buffer, it is critical in order to maintain smooth code to minimize anything that would distract and impact the data flow from your thoughts to your fingers.