Do not be dismayed at the following numbers! They simply reflect how much time it takes to finish documents!
Refinements such as a better sound card, better microphone, a custom vocabulary, voice coaching etc. may individually only make improvements that are difficult to accurately measure. A single change might improve accuracy from 96% (roughly 12 errors/page) to 97% (roughly 9 errors/page). This may not seem like much, but if one writes several pages per day this adds up rapidly.
Presume you write 5 pages per day -- very common for professionals. Reducing 3 errors per page per day means you have 15 fewer misrecognitions to correct. Presume it requires roughly 6 seconds to correct each of these. You will save 90 seconds per day -- or about 1/2 hour per month, 6 hours per year.
This analysis doesn't have much meaning until/unless you are already getting roughly 95% accuracy or better. That is one error in 20 words -- a long sentence. Below that, there is likely a problem either with an inappropriate vocabulary, a poor sound card or microphone, inconsistent microphone placement, machine noise, or poor speaking habits.
Another side benefit of improving accuracy is reducing frustration. None of us like to correct mistakes made by a computer program. But it is a fact of life that making corrections must be done. Do your best to reduce these errors by preparing your vocabulary, using a good sound system, and speaking clearly.