I play guitar, and I am certain that practicing the fine-motor sync skill of using both hands at once has helped my typing, and vice-versa.

For reference, my typing while transcribing rate is in the 100-110 range. My daily-use composition is likely half, simply because I spend most of my time composing in my head.

I will say that the big advantage comes from being completely comfortable touch-typing, without needing to look at the keyboard. Once you've achieved that, the mental load of typing fades in to the background, and you can spend more time considering content instead of the mechanics of creating it.