Mines goes 26mph (flat ground) with custom firmware, Zero motor swap and an external battery in parallel also used from the Zero.
Dual motor people can probably go faster, but I don't really see a point. Rarely do I reach top speed in actual use, and when I do it's scary lol. Dual motor swaps really don't make much sense in general besides the "look what I did!" factor. In real world use its extra speed you don't need while draining your range greatly.