I've only recently started thinking about porting. What got me started is interest in the .327 federal magnum. Looking at the Smith and Wesson 632 Pro, I thought to myself: "is it really necessary to port that?". Then in a discussion here, some folks talked about how:
1. Porting really is NOT necessary in this caliber. Even the lighter weight and shorter barrel of the J-Frame don't cause much trouble. Certainly something like the GP100 in .327 doesn't need it. (Of course, I'm open to different opinions from people with real experience here.)
2. The .327 magnum shoots out a ton of hot mess when it fires. A short barrel exacerbates this and the porting takes it a step further. Some Firing Line members actually said that aside from "tasting it" when they shoot, they've had adjacent shooters at the range relocate or ask them to shoot further down!
So, I'm guessing that there is a weight vs. caliber vs. barrel length relationship that determines when porting becomes helpful or optimal.