In the meantime, here is simple patch for case-insensivity.
You might be able to talk me into accepting various unambiguous, common alternative spellings of various units. But for instance allowing MB and Mb to mean the same thing is insane.
How would the docs for that look like? And anyway, what is wrong with Mb for megabytes?
From infamous wikipedia: A megabit is a unit of information or computer storage, abbreviated Mbit (or Mb). To me playing with case of acronyms and even depending on it seems more insane. It would make much more sense to have case insensitive set of acronyms and (thanks Tom for pointing out) some sanity checks when configuration is loaded to notify user when wrong ones are used for some context.
I doesn't seem completely unreasonable to me that we'd want to express something in megabits/second in the future. For example, instead of vacuum_cost_delay, it would be cool to specify a bandwidth allowance. Megabits/second is a completely reasonable unit for that. Or a limit on network bandwidth.
There are less confusing (better) acronyms kbit/s and mbit/s available for that.
FWIW, I don't feel very strongly either way. I'm more than happy with the status quo. The hint in the error message very clearly spells out what the valid values are, so it's immediately clear what you need to fix if you get that wrong.