I agree with using mirror extensions, just makes me feel better about what I can see.
As for the legality, IIRC from 20+ years ago drivers education a vehicle is required to have a min of 2 mirrors whether that is 2 side mirrors or a rearview mirror and a side mirror. I don't recall any specificity of field of vision required. In fact as a kid it was very popular to replace the "large" OEM mirrors with much smaller and sleeker looking side mirrors.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)