This area of discussion gets confusing in a hurry, in part because people sometimes use the word "focus" in one of (at least) two very different ways: 1) visual focus, which is a function of what objects or distance is sharp; versus 2) mental attention, which is what thing(s) the conscious mind is paying attention to.
I'm not sure a fiber optic helps with the former. Indeed, in bright conditions, a large fiber can be harder to visually focus on than a darker sight.
The real benefit of a fiber dot is that it allows some level of awareness of the front sight while shooting with a target focus. If you're trying to shoot front-sight focused at 15 yards, I don't think a fiber dot is going to change much about that. At 15 yards, the only real benefit of the fiber, IME, is that it may help you maintain front sight awareness during the recoil process a little better. If you're trying to shoot target-focused at 15 yards, which a few people can do effectively, a fiber is likely to be quite helpful. But I have never, ever heard anyone say that distance shooting got easier when they added rear fibers. That's not a thing.
I personally like a fairly small, green fiber on the front sight and a plain or serrated rear. My USPSA limited division gun is set up with a 0.040" fiber in the front and a plain black rear. I consider 3-dot configurations ill-conceived (with the possible exception of night sights for non-competition purposes), and a 3-fiber setup preposterous. This is obviously personal preference, but the vast majority of higher-level iron-sight practical/speed game competitors use something close to what I'm describing... at least 85+% of them run fiber fronts with a moderate to small dot and a plain black rear. This should be the first setup that anyone tries, and then, if they don't love it, do experiments off of that base. I would change the front dot color first, then it's size before I messed around with rear fibers.
Finally, as others have pointed out, fiber front sights are helpful, but only a minor improvement compared to plain black sights. Optical/glass sights with dots are where you will see a huge difference in speed and accuracy and ease of shot-calling. I don't know IDPA rules, but in USPSA, going to optics does put you into one of the optics divisions (open, carry optics, or pistol caliber carbine), so it may or may not make sense with your current gun. But when you are tired of struggling with the actual shooting and calling hits on far targets, particularly if your aging eyes are causing true focus problems for you, then it's time to get into optics.