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In my experience, seating too deep with pistols is a recipe for disaster.
For a simplified answer, on bottle-necked rifle bullets, as long as you are using a medium burn (or slower) powder, have not increased the powder charge AND the bullet is not jammed into the lands (i.e. has somewhere to go), the pressure curve will change very little. When the powder ignites, the first things happening are the expansion of the case to the chamber walls, this creating a seal. During that time, the bullet is also moving to engage the lands. At the initial ignition point, because the bullet has compressed the powder column, there will be a pressure spike, but not one that should cause spontaneous ignition**. Once it does, and the case seals against the chamber walls, your pressure rises exponentially.
Like @oremike wrote, the leader between the chamber and the lands has a significant effect on the pressure curve.
With all that said, barring a kaboom, more pressure will always equate to higher muzzle velocity.
** RE:spontaneous ignition. Powder burn rates and efficiency are affected by chamber pressures. Some old powders that are no longer made are notorious for this - they burn very cleanly until you exceed their optimal pressure, at which point they are prone to catastrophic ignition, behaving like a pistol powder at that point.
For a simplified answer, on bottle-necked rifle bullets, as long as you are using a medium burn (or slower) powder, have not increased the powder charge AND the bullet is not jammed into the lands (i.e. has somewhere to go), the pressure curve will change very little. When the powder ignites, the first things happening are the expansion of the case to the chamber walls, this creating a seal. During that time, the bullet is also moving to engage the lands. At the initial ignition point, because the bullet has compressed the powder column, there will be a pressure spike, but not one that should cause spontaneous ignition**. Once it does, and the case seals against the chamber walls, your pressure rises exponentially.
Like @oremike wrote, the leader between the chamber and the lands has a significant effect on the pressure curve.
With all that said, barring a kaboom, more pressure will always equate to higher muzzle velocity.
** RE:spontaneous ignition. Powder burn rates and efficiency are affected by chamber pressures. Some old powders that are no longer made are notorious for this - they burn very cleanly until you exceed their optimal pressure, at which point they are prone to catastrophic ignition, behaving like a pistol powder at that point.
