Well, I am not sure what motor you plan to use it on, but on my previous eMTB I installed the Speedbox for Bosch and it was terrific. Here in Canada we have a higher speed governor limit than other countries (20mph/32kmh) than the 15mph your have. I can't imagine riding anything with a 15mph limit to be honest.
I found myself hitting "The Wall" often, especially when traveling on the flats between trails and it bothered me enough to buy the Speedbox. I looked at all the various devices and chose Speedbox as it looked well made (which it is), was simple plug and play, and it didn't screw with what was displayed on my computer.
That being said, I sold the bike and bought another as I progressed my riding and am now limited to 20mph/32kmh again. I have no plans to de-limit my current bike mostly because I ride almost exclusively with other eMTB'ers and there is no benefit it travelling faster than them on the flats.
If most of my riding was on my own, I might consider doing it again
