In a harnessed seat, I don't feel that EPS foam throughout makes a difference, safety-wise. Basically what Splash said. The child's body is held into the seat by the harness, and doesn't achieve much acceleration side to side so even if they do "smash" into the sides of the seat, it's really minimal, since the harness is holding them.
For boosters, I'm more comfortable with full body EPS. Seeing my DD in her boosters, I can absolutely tell that if we were in a collision, her body would more likely "smash" into the sides of the seat since only the vehicle seat belt is holding her- it's just not as tight as a harness, kwim? That's one reason that the Monterey is her primary booster- EPS foam throughout.