I don't know about distortion, but the ports sort of act like mufflers in the sense that they cause back-pressure on the driver. The more total volume encompassed by the ports (in other words, the more unblocked ports you have), the less back-pressure there will be on the driver, and consequently, the more SPL (horsepower) it can produce.
The reason I mentioned total port volume above is because both the radius and length of the port matter in determining how difficult it is to move air through it.
