We tried the same thing in the last few years. Although we used repetitive flightplans that are only filed twice a year (one for winter and one for summer) we changed our cost index quite a bit from around 35 down to between 12 and 16 and were reminded to allways fly CI, which turned out to be around 250kias during descent (737 classics).
One thing we noticed right away was that the working environment became much more quiet and bearable reducing our stress level quite a bit, another thing was we got payed more since we our pay was based on actual block time.
Very soon our operation team had to change the policy a bit allowing us to use our own judgement for descent since ATC just couldn't really work with that, every descent out of cruising altitude actually reduced our speed immediately quite a lot (usually around 30 to 40 kias or .20).
Now we get payed a fixed amount for each sector regardless of how long it takes to get there and many of us just fly close to red line, who cares about fuel since we're purely operating as ACMI anyway.