My top speed is pretty much limited to how fast I can run - if I fall I want to be able to run it out. Anything faster than that is scary, and I personally don’t want to take that kind of risks.
You will probably find this thread interesting: Fastest speed on a unicycle
Note that 3 in the top 10 are ungeared.
Changing crank length is not really the same as gears, as it would still be 1:1. I seem to remember hearing about cranks where the pedal position could be shifted on the fly, but unfortunately I don’t remember where anymore.
Geared cranks has been thought of before, see e.g. Dakoroman Drive System Pdfx1.