A browser plugin API won't help.
Adobe still has to provide a useable plugin. Last I looked the flash player is still produced by Adobe, although they claim that they get help from Microsoft and not from Apple and Linux Distributors and are thus unable to provide performant flash players on anything but Windows.
From my point of view the flash player is basically an interpreter for a scripting language, so open sourcing the Flash player might help on Linux since there are lots of very capable programmers working on interpreters for several scripting languages. Those OSS flash players can then be ported to OS-X and the performance issue as well as the bugs will be solved.