I've seen them out on rides, faster than I would have expected, maybe high 30s?
The ones that really look scary are those bolens diablo with the rear sulky looking ski thing you sit on. They do like 40 and it looks totally mental.
Started working on the seat, ugh, cover is trash, foam is trash, wood base is trash. Haha. Gotta piece of wood to cut down and glued up the foam with 3 layers of carpet padding. I have scrim foam to go over the top so we'll see how it goes.