Just an FYI, your idea is actually exactly how AEGs with shorter barrels work. Barrels shorter than 500mm usually use cylinders with holes, essentially exactly what you propose. To get optimal performance, one must match the volume of the barrel to the volume of the cylinder, past the aforementioned holes. Check this out:
http://www.21stcenturyairsoft.com/cylinderguide.php
Someone who knows more will probably have to confirm/deny this, but I don't think acceleration of the piston is something that you should be concerned about taking a long time... It's my impression that what you're getting at with the holes in the cylinder is that it takes that much time for the piston to accelerate to terminal velocity. Even if it's pushing a BB, a piston starting from rest will accelerate extremely quickly.