In "Shabbos Kehalacha" (Chelek Aleph Perek Ches), Rabbi Yekusiel Farkash defines mitztamek veyafa lo as a food that the individual benefits from its continued cooking after reaching nisbashel kol tzorkoy, regardless of the benefit to the dish itself. He gives the example of meat that gets shriveled up and soft through additional cooking; if the meat will be used only for his family he would be happy to have it softer and it is mitztamek veyafa lo, however if he is expecting guests it is mitztamek verah lo since he wishes to give them presentable portions and not shriveled meat. He extends this to health concerns; if for medical concerns influence how well done the food should be, that will cause it it to be mitztamek vetov/verah lo.
In ha'orah zayin he deals with foods that are specifically mentioned in achronim to be considered mitztamek vetov lo, and discuses when an egg would fall under that category.
