Admittedly: I only skimmed the first page and read a handful of replies, but I get the premise.
First up, the damage:
If you increase the damage, you mitigate the hit registration problem. Understand that the Sting Ray isn't meant to kill; none of the new specials is meant to kill. Especially the Sting Ray. Adding more power on hit could turn it into a 3 hit kill, giving ample time for the opposing players to escape, ensuring the special is not overpowered.
If you increase the strength, the size of the beam and the turn speed should remain the same. These specials, the Sting Ray in particular, is meant to herd the opposing team, or pick up assist splats.
Next, we'll address the beam size and turning speed:
The turning speed, in my experience, is mostly mitigated by repositioning and firing the beam again. I have seen exceptional use of this special in such a way. Sure, re-firing the beam wastes special, but turning that slowly means you won't even get any range out of the thing to start; so those are your current options with Sting Ray as it is now: drop the beam and swim to reposition (wasting special duration) or selectively cover a single area/choke point with the beam's terrible arcing speed (essentially severely limiting the use of the special to very niche situations).
Imagine, however, if the beam size was doubled! Keep the power of the beam as is, but double the size and scope of the beam to ensure it is as threatening as the other specials, and almost as useful in a more robust set of scenarios.
Separate from that, consider speeding up the beam's movement speed, similar to the speed of the sniper mobs in salmon run. Same size beam, same damage output, but now it has a much wider range of uses from a much, much wider selection of map points! Speeding the special up this much adds great versatility!
Okay, but everyone's mentioned all of the above already. What's the end result, then?
An objective look at the Sting Ray, now and based on the above changes:
As much as we want a strong, bad-*** beam cannon to splat foes from a safe distance, the Sting Ray can never be that. If you add all of the buffs myself and others spoke about above, you create a special that fits in very well with the meta of Splatoon 1... but not the current meta of Splatoon 2.
With added speed, size, and power, you have created a special that, by its very nature, would destroy the opposing team; and, I get it, your counter argument would be that the special would be more balanced than that, and you're right, it would. However, I feel like any of the singular tweaks to the special would be enough to give it the competitive use it deserves.
Think about it this way: if it isn't broke, don't fix it. Rather, consider the purpose of the special as it exists now in the current evolving meta. There are methods to use it to a greater capacity than pointing and blasting, and it does have niche uses in every map. In this case, why not consider taking this purpose and applying it in a new, more unique way?
Imagine if the Sting Ray remained the same in terms of beam size and turning speed, but instead of doing damage, applied the Disabler effect (Sploon1) to those it hit! Slows the opponent down, shows their map location, boom. You just took the same concept for the special and reapplied it in a new way to the current meta. I bet that'd see some comp rotation on rain maker.