The problem is utilization isn't the only measure of efficiency. And efficient design is as much about avoiding stalls, bottlenecks and saturation as it is about high utilization. If the city planners in your town designed the sewer system to be at a high level of "utilization" on an average day, that might seem very efficient, but only until the first time it rained and every toilet in the city backed up at the same time and millions of gallons of raw sewage is dumped into the local waterways. In that case building excess capacity to cope with peak loads and worst case scenarios is more efficient, which was my point about the ROPs.
What happens if it floods once in ten thousand years? Efficient in what sense? Utilization of money? Maybe it's better to just let flood and rebuilt/cleanup. You see like SB was trying to explain to you. Efficiency isn't the same as capacity/performance. You are right, there are different kind of efficiency. The efficiency is in the context of utilization of something. So when we're talking of GPU, we're talking of keeping all those all those units utilized.
In the (supposedly -- it's all rumors) Orbis's case, it can handle peaks a lot better. But it's will spend higher percentage of its time idle. So those idle time is what is been referred as inefficient. But hey, being inefficient isn't so bad...because I rather drive Cobra than a Honda Fit (though I can't afford neither -- too expensive to own a car in the city).