The SmartSeat requires the harness to be below the shoulders (not at the shoulders) because the chest Gs were too high with it at. Most seats are fine with shoulders at the slots. Physics is the same everywhere, but the seats are designed differently. What works with one seat doesn't work with all of them.
It's my understanding that the primary reason they require the harness to be above the shoulders is due to spinal compression. We keep the harness below, and I don't know that we have any evidence that spinal compression is a concern with a RFing seat. We are more concerned about the child ramping up and out of the seat, especially since we don't have foot props to limit downward rotation.