I've measured the practical difference in the .260, 6.5 Creedmoor and 6.5x55 using water, with the same 129-grain Hornady Interlock seated to the cannelure. This is more realistic than filling cases to the mouth, which makes longer-necked cartridges appear to hold more powder. The results were around 48 grains with the Creedmoor, 50 with the 260, and 52 with the 6.5x55.
Since potential muzzle velocity increases or decreases at 1/4 the rate of case capacity, when loaded to the same pressure, this means the 6.5x55 has about a 2% advantage over the 6.5 Creedmoor, around 50-60 fps with most loads. The .260 is obviously in between, so there obviously isn't any practical difference between any of the three ballistically.
