Opening Gambit: A useless ability in my eyes. Unfortunately, its effects aren't that strong and it's not very rewarding for 30 seconds. I'd like to see it buffed, but there's an issue -- Run Speed Up and Swim Speed Up is actually programmed to cap out (like Attack / Defense Up). So the only way it could be buffed is if the time was extended to 1 whole minute instead of 30 seconds. It would actually be much more viable.
Last Ditch Effort: Same as Opening Gambit, except that Ink Recovery Rate and weapon ink efficiency doesn't cap. Since it doesn't cap, it could just have more ink recovery and efficiency, but it could also be extended to 1 minute instead.
Comeback: There's nothing wrong with this ability, but when you compare it to Opening Gambit and Last Ditch Effort, it's like both of those combined, but much better because it can activate multiple times throughout a match. If OG and LDE (abilities I mentioned up top) got extended to one minute, then those two would work very well together and can compete with Comeback.
Bomb Sniffer: It's a great ability, but when compared to Ink Resistance Up and Stealth Jump, it's probably the weakest of the three. I think this was mentioned in the OP, but if you took less damage from bombs with this ability, it would be top notch. To explain a little further, I think being right next to a timed bomb would still kill in one hit, but if your outside of the one-hit kill range, that's where you'd take less damage. --- Maybe to make it easier to program, the blast radius could be smaller for those only with Bomb Sniffer, thus giving the appearance of less damage.
Haunt: It has niche use to me. When doing Turf War, if you get killed by someone, they are very likely to be killed within a few seconds because a teammate would be nearby. This seems that Haunt is doing it's role, but is it? In reality, no. When I compare how likely someone else is to die after they've killed me, it takes about the same amount of time with or without Haunt... So Haunt is virtually useless in that scenario. Now, the only time it's beneficial is when you're on a large map, or in other ranked modes (Splat Zones & Tower Control mainly). I still think this ability could be buffed, but I'm not sure how... Maybe if the person who killed you is right next to their teammate, Haunt will affect them as well? Or maybe if two people work together to kill you (i.e both shooting at you), they both get haunted? I don't know, but it needs a buff.
Ink Saver (Sub): I thought this ability was great, until I used Ink Recovery in its place. Ink Recovery is so much better than this ability when it comes to using multiple subs, but why? Ink Saver (Sub) just doesn't save enough ink. Its been said that Ink Saver (Sub) decreases ink usage of your sub weapon up to 75% of its normal usage, but I'm very skeptical of this being true. To me, it's more like 25% of ink usage is being decreased and not 75%. I think this ability needs to be buffed in how much it decreases ink usage (i.e more percent).
Run Speed Up: For squids sake, please remove the cap on this ability. Swim Speed Up is programmed to cap like Attack Up / Defense Up, and it's slightly understandable, but I don't see the need to cap Run Speed Up when swimming in ink is a better tool to get around.
Special Duration Up: An ability that gets over-shadowed by its counterpart, Special Charge Up. After Special Charge Up got fixed via patch (it's always consistent when charging), Special Duration Up is left to crumble in the dust. This ability isn't that beneficial because the majority of specials, at most, last under 1 second longer with this ability. People may say that every second counts, especially in a shooter type game, but this ability doesn't even last more than 1 second, it barely reaches it. The main special that benefits from this is Echolocator, going from 12 to 14 seconds. Ink Strike and Killer Wail setup times also go up from 10 to 12 seconds, but that much time isn't usually needed in most scenarios. As for the other specials, well... you can forget about those since they practically don't benefit from this ability. In the end, this ability should be buffed, adding at least 3 seconds or more to those specials not mentioned.