That's odd... In my present airline we are using cost index 4 and it gives us a speed of 270 IAS
We used to do .78/300 for the longest time until the FDM showed using the managed 270 knots was saving fuel.
Also, increasing speed or decreasing speed for tactical advantage is alway benificial in a situation where it could enhance traffic flow.
For instance an airplane like q400 or ATR72 we would increase speed if we are at almost the same distance to land, minimising his delay and enhancing the traffic flow.
Hope that helps...