Wish I had a real answer for you, but here'* what I know about the UIM (besides the fact that it gets old and leaks coolant before you discover the joys of GM Forum and socket sets).
The runners seem to direct airflow through the UIM. However, like the MAF post in your throttle body, directing airflow means either reducing volume due to obstruction, or reducing velocity*due to directing. At high RPM'*, there'* enough air velocity that the runners actually slow things down (hence the existence of the HV3); however, at low RPM'*, the runners are needed to properly move air through the manifold; otherwise, the air would just slow down and stagnate. Proof: It'* been shown (by ZZP themselves, I believe) that completely removing the runners adds high-end horsepower but absolutely kills low-end torque.
Long story short, it seems doubtful that you can port the UIM to get more power out of it. On stock runners, I think reducing
its height and therefore shortening the path the air takes through the manifold would be your best bet. (But bear in mind that I studied computer science, not mechanical engineering.)
However, I would look into polishing. I forget what the inside of a UIM looks like, but like a LIM, smoothing out the air channels would help a tiny bit.