A 2 seamer is more a product of the hand position at release rather than grip or finger pressure. Putting extra pressure on the index finger doesn't make much sense because the ball is going to come off the middle finger last anyway.
I think some people try to make the 2 seamer move too much rather than just gripping it and throwing it. When the wrist is at a position of slight pronation at release, that's how the movement is created. Brandon Webb's sinker doesn't have that exceptional movement because of the 2 seam grip, it's all his hand position and getting "through the ball." Webb's 4 seamer would sink just as much as his 2 seamer because of this.