Well, you are already well ahead of me in the watts/kg department! When I did my first FTP test at 181w, I weighed 90kg, so just barely 2 w/kg. And now I am 84-85kg, so 316w is still just around 3.73 w/kg. I don’t think I can lose much more weight without dropping lean mass, which I am trying to avoid doing (older people should hold onto as much muscle as they can—as Peter Attia likes to say, no 90-year-old in the history of the world has ever wished they had less muscle). So any gains in the w/kg dept for me will probably have to come from more watts. I don’t know what the limit is, but I will try to get there 
As for your getting to 300w, that would put you at 5.3 w/kg which is pretty insane on six hours a week. Maybe it’s possible, but a multi-year process.