Speed boxes don't work with Yamahas. Know many people who have tried on the Yamaha brand ebikes, came away disappointed, and out the money they spent for one. It will also void your nice 3 year motor and battery warranty. (though they might work with Yamaha's motors on other brands of ebikes, but that's because those brands use 3rd party control software that probably dont have the same proprietary software. The OEM is doing this to protect you from doing stupid stuff to your controller and motor. ) Frankly they are all pretty stupid concepts bc you not only lose the warranty, and speed indication, but you also run the risk of over-amping the controller, blowing that out, and Yamaha and the dealer both will know right away you put a speed box on it, due to the software they use that records the last parts of every ride. Its also not a cheap repair to have the controller replaced in Yamaha mid drive. We've seen this discussed over and over again on the forum for years on how these mid drives slow down approaching 20 mph.
Crazy the topic keeps getting brought up over and over again. So buyer beware and buy what you really think you need from the outset, as these 20 mph mid drives all have the tendency to feel like they are not assisting all the way up to 20 mph. That said, I have yet to ride a CrossCore or CrossConnect or Urban Rush that did not assist solidly up to 21 mph.
Personally I would never want to buy an ebike that has a 20 mph limit, and its not because I want to ride at 28 mph, but my general ride cadence, and what I prefer to do in terms of gaining momentum before going up a hill, puts me at speeds higher than 20 mph. On an ebike, I like to cruise right around 22 to 23 mph on flats, because it gets my heart rate up where I want it, and I can feel enough in my legs to get the work out I want. On a regular bike I didn't need to get to that 23 mph speed, since I was doing all the work. I'd get the work out I wanted doing 16 to 18 mph on a regular (non electric) bike. If wanted to just relax and have fun, I'd hang below 15 mph, since wind has less of an effect, and the impact of wind is non-linear. (an exponential force function as you increase speed. Btw, if it wasn't, there would not be enough 'lift' for airplanes to exist.)