The gains are small, but do exist thanks to relieving the extra bit of restriction from having a cat to going cat-less. This has been posted repeatedly from independent sources and tuners.

My look at the whole thing is, if you're getting an aftermarket header with a high-flow cat, you're already illegal because you're tinkering with emissions related parts. So whether you choose to go with a cat-less or high-flow cat header becomes a preference thing. I went with cat-less because from doing enough reading over the years from peoples opinions and thoughts on this, I came to the consensus that there isn't much of a smell if any, and that there was slightly more gains to be had. Since I was sticking to NA, I made my choice. I have no plans to ever touch my front pipe, nor ever go to a high-flow cat in that department. The smell, alone, would turn me down.

I don't think the "primary" or "secondary" names have anything to do with whether one does more work than the other. Purely named for purpose of understanding their location in respect to the whole exhaust. At least that was my guess. Primary comes first, secondary comes second. I do agree with you on the sensor part and don't have a good answer for that. Tried looking the other day for info to backup my statement but had little luck finding anything at all. I'll post if I do.

The more research I do the more it seems you are right and I am wrong. It’s just so surprising that the smaller cat is the one people remove to make the most power. My 2001 Camry has the same setup (header cat + downpipe cat) and it functions in the way you described. Safe to assume the brz must function the same?

Still unsure haha. I did a bit of reading last night myself and could not find a reasonable verdict. Everyone has mixed thoughts on this. My thought on the primary cat removed making the most power is that it's slowing the initial flow down of exhaust gases unlike the secondary which is way down the line. But I honestly don't know much about any of that other than the fact that the flow of exhaust gases is complicated - hence why we can't just throw on a bigger exhaust or remove a cat and get instant gains necessarily.

In any case, the reason we got into this topic was because of exhaust smell. And I was saying that I don't get any smell once warmed up. I was actually thinking I'll experiment for fun when I get home one day. I'll idle the car for a bit, get out, and see if I smell anything and how bad it is. Because the truth is, I hate exhaust smell and I hate to put others through that as well. That is why I was very careful about choosing my exhaust and took a long time to decide. I was looking for best gains through the power band, little to no smell, OEM+ like sound quality and quietness. I've never had an issue passing emissions testing with my setup either for the past 3 years. I live in southeast PA, so YMMV.

Get a friend or family member to drive your car while you drive behind it in a different car with your window down. Only way to really experience/verify if it stinks or not.

Certainly could, but performing the simple test I described would give me most of the story. If it has little to no smell at all, that gives me comfort and anyone driving behind me should be fine as well. Granted when I'm giving it full throttle, there's probably some smell, but the reality of it is, the stock car smells at full throttle at the tailpipes as well.

I never said anything about full throttle pulls, but rather I meant city cruising (25-45) speeds.

Politely disagree that attempting to check for smell at idle will give you "most of the story."

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I think any exhaust mod is where I live. Luckily no one is checking.

Smelled many WRX/STI car that are catless .

I don’t think it’s worth it, you need at least one cat.

even if it s smaller, the primary cat is a lot closer to the exhaust valves and that surely affect performance more.. it s the first big obstacle the gases encounter so it s quite big of a restriction

the secondary o2 sensor is placed after just 1 cat because it also aids in correcting the afr via fuel trims during cruising, and the first cat is already more than enough for the o2 sensor to measure different from the first, so that the ECU thinks the cat is working
Also placing the o2 sensor after the second cat would bring it much further from the exhaust valve and also close to ambient air which then makes it useless to perform afr adjustment
the 2 sensors in oem tune try their best to keep afr at precisely around 14.7 during cruising because that s the afr that produces less pollution when coupled with catalyzers

When i tune this car, i disable completely the secondary o2 to the point that you can rip it off and the ECU doesnt care, and offset the primary lambda to get a real cruising afr of 15.5 or leaner, a thing impossible to obtain when on oem tune