I've never gotten anything to SuperFresh actually, and I think that it might be unreasonable to use that a metric for some weapons. Turf War being a more "Carry or get carried" (All Splatoon essentially until you start playing with teams) might leave weapons that fill a supportive niche left out. It also may depend on your style, I'm a fairly objective and defensively oriented player, which tends to leave my K/D in a stable but lower average state than most hard carry players. For Turf War, that can often mean simply not winning as much.
I think the best metric is just how good you feel with a weapon. In Splatoon 1 I was a master CDS player. I played the CDS (Custom Dual Squelcher) more than any other single weapon, and for a long time by a considerable amount. After a certain time spent with a single weapon like that, it begins to feel more like an extension of your character (Cheesy to say I know, but trust me) than a weapon you are playing with. I could feel the rhythm of its fire-rate like the beat of my own heart, it's range like the reaching of my own fist at the enemy. When I was exclusive to the CDS, I had a confidence in my worst element (Aggressive fragging) that I could sense in no other weapon. At this level of familiarity, how you're playing your weapon is second nature, and you begin to feel the match at large unfold around you. You no longer consider what angles are good or bad for you, where you can reach with your range. You don't need to think about whether or how you can tackle an opponent, you start to think about whether it's *important for you to do that* at the time. Strategy moves to the forefront of your mind and the mechanics take a backseat.
That to me is how I begin to know that I understand my weapon.
To add to this, one of the first points of advice I give to people who want to learn if to take one weapon and Main the hell out of it. Keep a couple back ups, but as much as possible stick to that one weapon, even when it gets boring, even in bad match-ups. For every hour played on not this weapon, play three on it. Reach that point where you stop thinking about your weapon ever and you just know it. When you get that far, you start noticing so much more about an individual match and you begin to learn extremely quickly for a short period before the finer elements begin to settle in. When you can eliminate all the variables of unfamiliarity or inexperience on your end (By using only weapons you are extremely familiar with) you can more easily identify what's wrong with your strategy, or adept to the strategies being employed by your allies and opponents. This is the path, at least initially, to being an extremely strong solo player and heightening your individual ability.
Edit: Sorry, didn't mean to get all deep on ya, it just sorta happened while I was thinking about it.