Did you try the '350w display' with the 15a controller? like op may have.
Yose-Power may compound the issue by selling 2x brands of 15a controller, with the cassette kits its a JYT brand controller supplied, this may be the one with problems??
Its the type i have and after reading about issues i dropped my interest as by then the bike was on the rd and i was More than content with the speed being limited at 15mph .. MY bike is not built for speed..
No, because I had already decided that I didn't like the abrupt cut-off that the JYT 15a controller gave, so it was a case of "try another controller". The 18a "ramps down" at the limit of each PAS setting much better.
As an aside, like many others new to e-bike kits, I got hung up about more about speed, but as time has gone on I find that I am happier at sub-15mph most of the time. The cold air doesn't make my eyes water, if I use a lower PAS level I get more exercise and the range increases. About 10-12mph allows me to look at the scenery.
I've even changed the throttle setting back to the 6kmh limit as I only really use it as a "nudge" to get moving from rest if I'm lazy or in the wrong gear! My other bike is fitted with a Yose 350w rear freewheel kit and I now have it set up in the same way. At 17a max setting the only real differences are that due to the motor gearing and /or windings, the 350w accelerates quicker, but the 250w is slightly better at slower speeds on steep hills.