Well, the way I see it, the difference between 13mm and 12.5mm is 0.019685 inches. But that is a diameter, so you'd only see half of that disappear in the rollout by having the tip reduced to 12.5mm. Let's call that 0.01 inches just for ease of understanding.
If that tiny amount of rollout (currently close to .02 inches at the max) bothers you, I'd say forget about that purchase. The way you describe the rollout, it is .02 inches with the peak around 14-15 inches from the tip, so well within the stroke distance. Is your stroke repeatable to within .02 inches (or less) on every shot? That's 0.039 tip widths (on a 13mm tip).
There's no doubt your mind is half or more of your game, that's very real, so that is why I say move on to the next cue.
But as always that is simply IMHO.
Gary