It sounds intuitively right that cats are happier when they can hunt outside, but it would be interesting to see research on whether, and how, a cat that is kept indoors permanently, can still be happy.

Indeed. I can only attest from my own experience, as well of that of friends'. Including cases where a cat was thought to just have an irritable, discontented character, until they gained access outside (whether by the humans moving houses or the cat shawshanking his way out), and were then found to actually be happy, relaxed kitties.

Cat "gurus" and the likes have their recommendation as to how to keep your cat more content while still confined, but in my opinion, it will never fully give them what they need. Same as I could never be truly happy without going outside, no matter how grand and well equipped my home might be.

Re "quick suffering": I do not mean it is instant, only that it is still a matter of minutes (it is rare that it is over 10 minutes, and most times would not even get to that), which is infinitely less that the whole lives of animals in the meat industry.

Re not reducing demand for store-bought food, I am not sure I follow. What I say is that when a house cat also hunts and eats wild prey, they eat less of their store-bought meat. 

Does that answer your question?
 

I think this is what all cat owners in the world should have done, regarding the overal suffering owning cats causes - Cats cause a lot of suffering.

Do you estimate that in most of the world, cats are bought? Meaning, that more adopted cats mean more cats are bred to supply that demand? Otherwise, it shouldn't make a difference. Quite to the contrary: a house cat that also goes outside does hunt significantly less than a stray cat (though the strays to have much shorter life spans...).

In Israel, where I live, it is definitely not - but I know it is not the case for all places. The vast majority of house cats here are stray cats that were adopted, some of them when they were found as deserted newborn (now there's an EA dilemma: is it indeed an altruistic act to then adopt the newborn, and spend the vast resources to raise them into a then-hunting cat? ;)).

I know that in Berlin, for example, cats are normally bought and paid for, and also pure-bred (which in Israel would be considered a very immoral thing to do).

What is it like where you live?

It seems that we strongly differ on the importance we give to "chronic" vs. "acute" suffering. I put much more emphasis on the first one (though both are, of course, important). And while cats do often toy with their prey, it usually is just a matter of minutes, not sure if that is what you referred to.

Re mice vs. chicken: that is a good point, I haven't thought of that. 

Re your calculations: have you read the post someone linked to in reply to one of your other comments, specifically about the suffering cats hunting causes? While I didn't read it thoroughly, it is very illuminating, and might change your mind on the number of bird/mammals the average cat hunts.

And while it is but one sample, I can tell you that my own cat (that is very active, has access to wildlife and is a fairly good hunter) definitely does not reach that number. Most days it is just insects. A small mammal (usually a shrew, which I hate seeing as they are adorable) - possibly once every couple of weeks, recently? And until a few months ago, none, so I guess it also varies depending the season and weather.

However, I absolutely agree that cats should be spayed or neutered, as there are plenty of rescues that need a home. This is especially true where I live, where there are perhaps 10 stray cats on each house cat (in case you were worried my cat going outdoors is damaging some fragile equilibrium).

That is a bold statement, which I believe is also false. Do you have any support for that? As far as I can tell by observation (as well as common knowledge), cats do eat the vast majority of their prey (and in cases where they don't, there is likely to be some illness with the prey, which also means a swifter death is better for it).
 

Cats love veggies too, though. My cat happily munches on sweet potato, and once got my entire lunch on the floor trying to get my avocado (she succeeded :( ). I use seaweed as treats for training her, and when I sprinkle nutritional yeast on my meal she would not stop nagging until I give her some, too. 

Point is, it's not that the only taste cats interested in is meat. They have a diverse palate :)

Re keeping cats indoors: I believe that all animals deserve to be free, and more specifically, that I, as the cat guardian, do not have the moral right to them confined*. I would be interested to hear your counter-arguments, if you would like to share.

* (Unless absolutely required for their own safety/well-being; I would keep a cat indoors only if I lived near a busy road, for example. Though in that case, I would strongly reconsider getting a cat in the first place, or make sure that the cat is an older one, that does not need to venture out as much to maintain their mental well-being).

There are statistics showing that cats allowed outdoor kill mulitple (>1) small mammal/bird per day, and often in slow and torturous ways, which makes it very easily more suffering caused than their meat consuption in a day if they consumed land animal meat. 

I think the opposite: since that outdoor prey gets to live its life freely until that moment, those few moments of suffering are nothing compared to the suffering of an animal in the food industry, that spends its whole life caged and handled. 
I also factor in the bigger animal's much higher capacity for suffering.

I see it differently. Hunting and playing with their food, while unfortunate, is their natural tendency - and unlike humans, they can't do better.

I sadly feed my cat meat-based food - there is no reliable vegan option available where I live. When she does go out and hunt (and I am an firm believer that cats, as all other animals, deserve freedom), while it is often upsetting to watch, I know that the quick suffering her prey goes through saves heavier, more prolonged suffering of the animals her store-bought food is made out of.

It's a question that bothers me since I got my cat. As a long-time vegan, I hate that I buy animal-based food for her - but I couldn't find a reliable vegan option. 

However, I always thought about it as a minor problem in the grand scheme of things. If cat consumption of animal meat is actually significant, I see that - bear with me - as a cause for cautious optimism. 

As cats don't have the same psychological hang-ups on food as humans do, I believe it should be easier to make the transition to plant-based food. It does require, as you wisely mention, more RCTs and research in general, so cat guardians can ensure the long-term health of their kitties.