105 BR-5500 an improvement over Sora BR-3400?
Would 105 BR-5500 brakes be much of an improvement over what I have?

What could the improvement be?
Lighter weight would mean the brake arms are less stiff, causing flex in the brake system. Thus, you cannot reasonably reduce the weight of the brake arms. Besides, most weight reduction &quot;upgrades&quot; are not worth it, giving only few tens of grams of savings at most, the benefit of which is infinistesimal but occurs at great cost.
Stiffer brake arms (less flex) would mean the brake arms are are heavier than necessary. Weight being unacceptable in a bicycle, the manufacturers are reluctant to do this.
Higher mechanical advantage (more power in brakes) would mean the brake lever runs out of travel very quickly, requiring frequent turning of the barrel adjuster, and the free spacing in the brake pads is so low that they cannot track a wheel with a broken spoke. However, do note that current trend of high mechanical advantage in dual pivot brakes with their forced centering and current trend of less than 36 spokes per wheel may mean you may run into problems if you break a spoke anyway.
Higher free spacing (allowing to track a wheel with a broken spoke exceptionally and not running out of lever travel quickly) would mean the brake has little mechanical advantage and would require unacceptably large lever force to stop.
Sometimes, a design is a compromise that has been perfected decades ago.
If you want an improvement in brake technology, there are three options (or actually two really as you cannot find hydraulic rim brakes):

Better brake pad material. The Kool Stop salmon-colored compound works exceptionally well in varying conditions, not picking up pieces of sand that work as an abrasive compound to cause early rim wear. This is the cheapest approach and I highly recommend it.

Better actuation mechanism. Cable actuation can be replaced with hydraulic actuation. However, you probably won't find hydraulic rim brakes easily.

Better brake type. Rim brakes can be replaced with disc brakes. However, disc brakes are heavier (because they do not reuse the rim as the brake disc but rather require a separate brake disc) and require a heavier fork and a more sturdy spoking pattern in the wheel. On the other hand, disc brakes do not have brake lag in the wet, unlike all rim brakes. The cost is more frequent brake pad replacements. I don't recommend this approach because in practice it requires getting a new bicycle, as forks not having disc brake mounts cannot support disc brakes.