It's an enhancement designed to protect the gun fromtge elements and also makes it run more smoothly. So I believe it increases the value in that aspect.
The 25/28 Glocks aren't imported because they don't pass the ATF checkpoint system. They G19 barely passes and the G26 passes with exactly enough points. If the 25/28 were 9mm, they would pass, but since they are the smaller .380, they fail.