In that case it sounds like you're volt or watt limited or a combination of both. One option from here is to try a higher voltage battery.From reading those instructions I got in to the speed settings of the controller but it's already set to Max 40kph but tops out at 25-27kph with no assistance after that speed. . Strange. I have adjusted the wheel size but although the speedo reads faster and slower depending on size the wheel seems to spin at the same speed, haven't confirmed this with a separate speedometer but just by eye.
You can do a cheap test by getting a 6s lipo pack and wiring it in series to your existing battery and see if road speed improves. If it does, then you know a 48v battery is the solution and will increase your top speed by around 30%.
You might be also be watt limited, which sounds likely as the no load speed increases with wheel size changes but load speed doesn't change. You could try opening up the controller and soldering a third of the shunt to increase current delivered to the motor. Although the advantage of trying a higher voltage battery in the first instanbce is that it will also increase the current delivered without having to solder the shunt.