To go back to the original question: no, don't know if there were any changes to the '83/'84 motors. Where did you get those figures?
To respond to the side discussion on MPG: on my stock US-spec '87 motor, I get 24+ mpg on the highway at a steady 70 mph. If I bump it up to even 80 mph, it drops down to 20-21 mpg. Spirited street driving will see it plummet to 12-14 mpg.
1987 Venetian Blue (really looks like grey) 930 Coupe