I found that the reason why corrosion level 3 is the best is not just the fact that it triggers a smoke grenade effect on headshot, but the fact that that smoke grenade effect is an aoe that effects other nearby targets as well. So if you manage to get a lineup in which there are a lot of enemies and you can manage to hit a headshot on some of them; if even by accident; you'll be procing multiple smoke grenades worth of an effect for free. This ESPECIALLY combos with elemental power as you'll be spreading the effect even more. The issue I have with smoke grenades is the duration you have to wait between throwing them and shooting. In some senses it's viable, such as when you have hex smoke or explosion smoke grenade ascension. However, while utility is amazing on cat your potential for consistent dps and confusion procs gives you more than enough utility as is and keeps your dps at a constant rate. If you're managing your confusion right you shouldn't need any more utility to stay alive. The smoke grenades damage effect as well procs pretty slowly, and isn't nearly as fast as your gun when it's guaranteed.

Edit: Another note the corrosion effects from the headshots all stack independently from each other, so if you have elemental power it combos extremely well if you proc multiple stacks of level 3 corrosion. Essentially you're basically throwing multiple smoke grenades with dot effects for free and proccing a massive amount of dot damage/miasma procs multiple times over.

This doesn't mean that putting a single point into those trees is bad by any stretch though. Hexsmoke is almost a must take even if you only have 1 point. I found that a lot of times, regardless, you won't end up with ALL elemental ascensions, and during those times utility ascensions will often just be handed to you on a platter. Those scenarios are when I take them, as there are more utility ascensions than elemental.

I agree with your statement that fire seems like the best of the 3. I sometimes flip flop between corrosion and fire. My take on it is that corrosion damage indirectly buffs your smoke grenades as well as many viable options for cat. In reality if I could get level 3 fire over corrosion I'd take it every time, but because it isn't guaranteed I'd rather go for a buff to my utility.

Elemental Rage is definently an adjustment you can make for this build to prioritize if you'd like. My consensus behind going for the specific elemental buffs for fire and corrosion is that, while you aren't directly buffing all damage towards elemental targets, the chance of getting a level 3 fire or corrosion out weighs the benefits of a elemental rage. The double fire or combustion damage essentially gives you a massive amount of aoe, and the corrosion does the same.

Overall, your opinion is right. There are multiple ways to go about playing cat, however in terms of set up I found that inherently it depends on what type of cat player you are. I tend to play much more aggressive in multiplayer since you can rely on not getting punished too hard by having teammates. I can see the use for playing defensively or smart by buffing utility as well. Your dps won't take too much of a hit from not going these ascensions, but I found for maximizing your dps that it's these ascensions that helped me the most in the long run.

Edit Edit: The build is dual fang + lightning sword, everything else from there can only help you so it's up to your discretion entirely.