I think so, but the effect isn't that dramatic. When the upper jaw is impacted, the lower jaw should rotate up, causing the lower lip to rise. This would help reduce lip incompetence, which might make the upper lip look shorter because it doesn't have to strain as much. Also, upper jaw advancement can decrease the nasolabial angle, causing the upper lip to appear a bit shorter, and can also elevate the nasal tip, causing the nose to appear shorter.
A more dramatic way to shorten the midface is to bring the lower jaw and chin forward. This should make the face straight instead of convex and therefore reduce the apparent length of the middle portion. I too was worried about the length of my midface, but it seems that bringing out my upper and lower jaws will make it fairly balanced.