You could push a hybrid case on the Grendel, with an exotic materials bolt and approached 6.8 x51 kinetic energy.
Not even close. The 6.5 w/ 130gr from the same 16" barrel is ~2300fps. Nowhere in the ballpark of the 6.8x51 140gr 3000fps. Grendel is roughly HALF the energy. Grendel can't even use the same pressures as 5.56 due to bolt problems. It a dream to think you'd be able to double the pressure even with exotic materials.