I am not sure about the numbers but potential is there! However, I personally don't think bitcoin has matured enough to become a suitable choice for retirement funds yet.
Probably millennials who have recently started working may consider it but definitely it is not a suitable investment for middle aged people. They simply can't take such risks. Probably small token investments can be done, but definitely not as a retirement fund.
People who are approaching retirement age, probably would like to stay away from such a volatile asset. They would want a stable return every month and bitcoin certainly doesn't guarantee any return unlike banks!