Although just a "tich" shorter than a .260 Rem, it produces the same ballistics if loaded to the same pressures.

Whatever it does, the .260 also does. The 6.5x55 in a decent action and same barrel length, will actually best both by a small margin, due to it's 'slightly' larger case capacity.

The 6.5x57 has the same case capacity as the 6.5x55. I've read that is the reason for the slightly shorter case with slightly larger base - to have the same capacity as the 57mm case, in the shorter action (M96 vs. M98) which would allow the use of the longer 156gr. FMJ of the time.