Even in later works with more Bronze Age influence like
All-Star or
Superman At the End of Days, there's always a sense that Superman isn't
pretending to be Clark, he
is Clark, even when things like the slouch and the higher voice are clearly affectations. In neither of those works is our hero actually capable of articulating the relationship between Kal-El and Clark Kent. It's like in Tim Burton's Batman where Vicki is like "so, why do you Batman" and Bruce is like "er, uh, well, I guess it's just something I have to do." Mrs. Nxly asks Clark if he's Superman or Superman is him and Clark's just like "let's change the subject". And it totally works, you know?
I think that Maggin's worst moments portraying Clark Kent are when he tries to say "Clark is Superman's uncontrollable passion" or whatever outright with all this detailing on the psychological inner workings. The best ones are when he just lets the connection between them exist, like when Clark goes to Lex and offers to tell him his big secret as collateral for Lex changing his ways, or when Superman thinks about the day he was outted as the day Clark Kent died. Also, minor quibble, but I wish Maggin had called Superman "Kal" occasionally. After all, it's not like "Superman/boy" is really a name, you know?