There are alot of components that go into how a cartridge performs ballistically. Case design can have a huge effect on how the cartridge performs. In the case of the Grendel, the case diameter and shoulder design balances very well with the burn charachteristics of a couple popular rifle powders.
Another aspect is the performance of the projectile. Some of the factors governing performance are bullet weight, velocity and coefficient of drag. Moving the projectile faster means it drops less in a given distance- flat trajectory. A bullet drops at the same speed whether dropped from your hand or propelled to 4,000 feet per second. It's just that at 4Kfps it doesn't have time to drop as much. A low coefficient of drag means that the drag of the bullet is lessened, allowing it to retain more velocity. And velocity is energy. Mass is also energy. So when you put all this together, the 223 moves very swiftly and has a low coefficient of drag, but since it's only 55 grains in weight, its energy at distance is much less. If you go to the 7.62, the CD is up substantially and speed is down, but energy remains high with a typical bullet weight of 168gr. The 6.5 moves almost as fast as the 223 and at 123gr has substantially more weight. So you get a good, flat trajectory and good energy at long distances.
Another aspect of CD and energy is deflection. Unless you're at an indoor range, you're dealing with wind and thermals. And those two can greatly affect accuracy. In that respect, the 6.5 is a solid performer. While CD is important, bullet profile is the big one here. Big, fat bullets are easy to blow around. So are light one's. The 6.5 is a good balance.
I've been playing with my own 6.5 for a couple years. It's base is the Remington 300 Short Action Ultra Mag (SAUM). I necked the case down to 6.5mm and messed with the shoulder a little bit. I chose this base as I wanted to try it in an autoloading rifle and the AR-10 platform would accommodate the cartridge length. So far I've been astonished with the performance. The ballistics absolutely blow the 7.62 away.