We have had this discussion within our company. Problem which you run into as a pilot in this discussion is that you are only looking at one specific factor (in your case fuel consumption) and that is only one part of the equation within the cost index formula. Cost index is defined as:
CI= time cost/hr divided by fuel cost/hr.
The time costs are referenced as: 1) Fixed costs per trip (e.g. handling costs). 2) Time-related costs. These can be split in A)Commercial factors (e.g. early/late arrival, etc). B) Crew costs (longer or shorter flight time can influence the work and rest schedule of crews, etc). 3) Maintenance costs, these can vary significantly with small cruise speed variations, because most maintenance is done on the basis of condition and cycles rather than hours. A lower CI would lower the temperatures within the engine, increasing maintenance intervals. Also guarantees or contracts with the different OEM can influence your choices.
I agree that the fuel cost/hr has increased its presence significantly within the formula making your statement logical. But as a pilot I do not have enough information to say the economics are always better by increasing your normal CI to the 150-180 range (ours is 100 btw). I have posted a question to our company how often the data is updated within the CI formula (awaiting an answer).