Good discussion and some thread creep. I'll try and address the issues in the last few posts.
First PDA and understanding weight effect vs. engine/airframe deterioration. If the correct GW is in the FMS then APM type software will derive good PDA values for the cruise phase of flight. If however, extra load is put on and not accounted for in the ZFW and GW then the PDA will be skewed by the weight difference. As discussed previously APM PDA values are not the whole picture and from a flight planning system perspective a PDA value that represents the whole flight is more beneficial. It is important to remember that there are two PDA values; one for the flight planning system and one for the FMS. These two values can be from 0-4% different. It is also important to understand that the flight planning system PDA will also contain a component that matches the flight planning system calculations to the manufacturer's calculations. This is a fixed value and varies based on flight planning system provider, aircraft and engine type.
Next there is the issue of comparing what other operators use for CI values. this is a mugs game. Many variables enter the equation for calculating CI and no two operators have the same cost structure. The next issue is are you operating in metric or imperial units - their CI values are different. lastly comparison would assume that all airlines used the same, robust process to calculate CI. That is the furthest from the truth.
Then the great issue of tactical speed management and use of CI. This is a great theoretical idea, but doing it successfully is very difficult. First, we must realize that just increasing CI while maintaining altitude is a very costly way to reduce flight time. CI was designed to be used in a 2D (altitude and speed) solution. 2D solutions work in flight planning systems that have lots of data on wind profiles and computing power. Current FMS hardware/software does not have this proper 2D capability and was really designed as a system to allow the autoflight system to strategically fly the aircraft efficiently based on a 2D optimized flight plan with a given CI. we must also understand the effect of wind on speedup/slowdown capability and realize that they yield completely different orders of magnitude results in a headwind vs. a tailwind. You also need to clearly understand the cost penalty for the late operation vs. speeding up. I have seen pilots speed up a flight and spend five times as much as what was saved.
In a perfect world the best way to make the speed up decision is to have an in-flight flight plan generated to achieve the desired arrival time. this will adjust the vertical profile as well as the CI value to obtain the most speed up for the least cost. The incremental cost increase should be compared to the delay cost. Modern flight planning systems can do this very easily. LIDO has a module called VSOPs (Variable Speed Operations) that allows entry of a desired arrival time.
The issue of CI and efficient flight planning and operating is a complex multi-layered problem. I have been working in this area and teaching about it for over 10 years and still have only scratched the surface.