I also found that the microphone makes a big difference - with my Plantronics Voyager Legend bluetooth headset, it was basically unusable, misunderstanding almost everything I said. But if I used a cheap Logitech USB headset that I've had for a decade, alphabet accuracy was good.
Something else is that it does seem to struggle a bit with my accent. For example, with the alphabet I would say "air", and 75% of the time it would hear `oh`/`near` - however, if I said "air" in an American accent, it heard `air` correctly every time.
Will be interesting to see how your new engine fairs when it's released.