I believe Storm 2 is only geared for 12mph in the current arenas, and that it was only geared for 16mph in Robotwars. You don't need a really high top speed. you just need to get something pretty fast in a short period of time.
This can be changed with bigger/more powerful drive motors but even then you shouldn't need to exceed 15mph and even that is pushing it. I'd personally go with 12mph like Garfie.
I currently use this speed calculator - https://wahiduddin.net/calc/calc_speed_rpm.htm - Warning: The wheel size is in inches and it wants the radius, not the diameter.