True, it is not a myth. But it is not the same in actual testing on a motorcycle, as the raw data would have you believe.
The two different LiFePO cells we've tested act differently than lead-acid cells at temps down to 0° F in our testing in motorcycles.
All batteries lose output when cold - the chemical reaction is slowed down.
In back-to-back testing of a 9ah Shorai against a 10ah AGM Yuasa on a CBR600rr and D675, the lead-acid AGM cells seem to drop more in output (measured by testing starter/engine rpm) than either the A123 Systems or Shorai LiFePO cells at 0°-32°F (as cold as I can get our freezer). If you give the LiFePO cells a few seconds with load (lights on) before starting, their internal temp rises slightly, and they start even better (faster starter/engine rpm). Do this with lead-acid, and the output drops even more.
Because of the results of our testing that we decided to become a Shorai dealer. If they had not faired as well, we wouldn't sell them. That said, since Shorai batteries have only been available since late 2010, the one thing we cannot know is long term life and durability. We are hoping they hold up similarly to the A123 Systems cells that we have been running for years, and I see no reason that they should not. But only time will tell.