csl,
its how and where you drive. Rotaries do quite well on cruise. Its the start-stop that really kills. In comparison, a 100KM straight drive without stops would show more than 10KM/L even if you accelerate from 120KPH to 160KPH a couple of times - especially for RX8.
But the moment you toss in 10 start-stops - dropping to a complete stop and re-accelerating, the fuel consumption is really going to drop drastically especially if the right foot is very heavy on the acceleration.
For the sake of comparison, if you cruised a Perdana V6, the FC would easily be 15KM/L or so - a 50% improvement over its mixed city drive of 10KM/L.
But again, I wouldnt compare an FC-FD-SE against a Perdana V6. Different class, different target market.