cam selection/cyl. pressure

Yes and no. Depends on the given combo and rpm.

More duration usually means more intake/exhaust overlap (both open at the same time). This CAN bleed down cylinder pressure, especially at lower RPM.

Increasing cam duration can increase power but it is usually due to increasing the volumetric efficiency. Or simply the ability to fill the cylinder with more air/fuel mixture. Ideally for every combustion event, the cylinder would be fully filled with the air fill mixture equal to volume of the cylinder when the piston is at bottom dead center. For many normally aspirated non-race engines engines the volumetric efficiency is between 80 to 90%.

Free flowing intake, heads, exhausts will help with volumetric efficiency too.
Especially when pressure waves in the intake or exhaust are in sync.

The increased intake/exhaust overlap allows the intake charge to start escaping out the hot exhaust (pressure bleed) but it also jump starts the charge's velocity. Now that the intake charge has some velocity, a reflective wave from the exhaust will then slow the intake charge's escape out the still open exhaust valve and instead direct it to the rapidly increasing volume of the cylinder (as the piston goes down - vacuum). Once the piston reaches bottom dead center and starts on the compression stroke, the intake will start to close but due to the velocity it will continue to "pack" the cylinder even though the piston is starting to move up. This will effectively increase cylinder pressure.

When the combo is right for a given rpm, a normally aspirated engine can exceed 100% volumetric efficiency at certain RPMs - a ram air effect.

This phenomenon is why going longer duration often makes the engine softer down low but make more power up top.

As always, it always depends on the engine's particular combo.