I tried BTT so I could set my trackpad speed above the maximum setting on the 2018 MBP (High Sierra). The BTT appeared to have the same range (maximum 14). I tried the terminal line "defaults write -g com.apple.trackpad.scaling 25.0", for example, and after a restart the trackpad does seem to go to the set speed. What I see in the BTT after this is that it will show the set speed as the new maximum of the range, but when dragging the slider it will immediately revert to having the Macbook's maximum as its own.
Is this how BTT is supposed to work or did perhaps the input terminal lines mess with it somehow? Does anyone know a better way to go about this?