I've switched to a Kilby-made cue with a laminated conical taper shaft and 10mm tip for seven or eight years. Besides the conical taper and 10mm tip, Ron's cues have a thicker butt than modern cues; they're much like all cues were 50 years ago, and like carom cues still are. It feels much "steadier" in my hand than do modern cues.
Essentially immediately my play improved. Most noticeable in the sense of being quantifiable was a drill I use to keep my stroke straight: a version of shooting straight in shots on the long diagonal and try to have the cue ball follow the object ball into the corner (I vary that sometimes by following only to a specific diamond line, or only a ball width, or stop shots). The goal is to make the object ball 50 times consecutively and keep track of how many times I reach the other portion of the goal (i.e., follow the ball in, etc). Within a week or so, my success rate on making 50 balls consecutively went above 90% (from just over 70%); the other parts of the drill showed improvement in the 10% range.
Unfortunately for this discussion, going to the conical taper and 10mm tip weren't the only changes I made then. I also started using an open bridge almost exclusively, and started looking at the cue ball on the last stroke instead of at the object ball. I don't have any way to quantify how much of the improvement was due to any one item. I feel a good part of it was due to the shaft, but how much was the taper and how much the 10mm tip, I don't know. I know that the taper seems to guide my vision to a point on the cue ball, but the 10mm tip lets me see closer to the tip-ball contact point, and I don't know which is really responsible for that feeling.
One thing I do know is that the conical taper felt right instantly, even with a closed bridge, and that using a shaft with a modern taper now feels so awkward that I will rarely do it.