So.... after doing a bit of research and a conversation or two...
As I said, and you confirmed Briggs Poor, there is no SFI 57.1 spec. This "spec" actually never got past the suggestion/conversation stage.
It was also initially intended for motocross and specifically pertained to neck collars not chest protectors.
As for the Leatt Fusion passing "a more stringent testing protocol than the SFI standards for chest and neck protection" I'm sure is a statement coming from Leatt Corporation. The testing protocol that they are referring to I would bet is the CE certification - which is a European standardization of testing..
As for it being a more stringent test, I could very well be wrong, but doubt that the CE test requirements are more demanding than the SFI methods.
Keep in mind that in practically all the European countries, chest protectors and neck braces are optional in most, if not all, kart racing series.
Why would you have a more stringent testing method for something... that wasn't even a requirement?
By the way Briggs Poor... thanks for being a customer and taking kart racing safety seriously.
Thanks for the comment Familyman!