From what I could tell on mine, its almost all the way drilled out to the hex key end.
You can tell a difference between one that is drilled and one that isn't if they are in the palm of your hand. However when on my T-2 I can't really say I feel the weight difference.
Over the chrono it was slightly more consistent. I just like the fact that it stores some air instead of being just solid stainless. I could see that helping out during periods of autotriggering to keep the shots a little more consistant.