Someone was nice enough to document all the important stuff(power draw/efficiency) in some of his videos of his alternator-powered mid-drive electric bike build:
If you look closely, it's pulling over 2000 watts just spinning the tire at full speed with no real load on it!
Another video he shows the numbers comparing the efficiency of it vs a hub motor; the alternator bike uses about 50% more watt-hours than the hub motor for the same speed.
Yeah, sounds about right.
Then again, maybe not...looking at his chart of amp draw vs speed with no load, he's pulling about twice the current I was with the same test; his alternator must be even less efficient than the two of mine.
Even so...if range and maximizing your pack size/weight are important, this probably isn't the way to go. There are better options, though not nearly as cheap, or likely in as small of a package.
It would do well when you want a lot of power in short burst, or if you kept the power demands reasonable most of the time by keeping the RPM down. You do get some pretty impressive torque, so you could get away with keeping the speed (voltage) down, run it at low RPM and not waste as much power just making the motor idle. You could also run it through the gears and optimize the output you need by down shifting for hills and up-shifting for speed.
Not a complete disappointment. Glad someone documented some of what I was looking to find out, before I put in all the same work. It still applies for what I intended it for, though cutting my battery range down by 1/3rd might limit it a bit much. Decisions, decisions...