I wanted to do a separate post to summarize what went on. So in short..ish
My 2nd Gen 2005 GS300 was down on power and I concluded the catalytic converters(cats) were clogged. On the GS there is 3 in total, 2 welded to the manifold and one welded at the end of the mid-pipe. I stared with the first two, so I bought a cat-less test pipe from eBay (check the original post below) I left the 3rd one on. Removing the first two didn't fix the problem, so the 3rd one was clogged, upon taking the mid-pipe out the cat fell apart in its housing. I decided to leave the empty housing (bad idea as you'll see later on) I then bolted everything back together.
I go for a drive and the GS had a horrible raspy/screaming tone to it around 2800-3500 rpm. In attempts to fix this I removed all heat shielding, wrapped the exposed break lines, tightened every bolt in the exhaust tunnel, removed my heat wrap(in case the ties were rattling), re-welded the OEM transmission brace for the manifold, I did everything I could think of.
The last thing...I talked with my step father who had a similar issue with a Infinity G35, and he said the empty cat was causing "negative pressure waves" to travel back up the exhaust and resonate the manifold. So I cut out the old cat housing and welded in a 2.25inch pipe(AutoZone). This fixed the issue, now the GS has a nice low tone when accelerating, the rasp comes back around 4000 rpm but it is very mild.

Check out the my original post for the steps/conversation/troubleshooting I went through.
If you have any questions about headers/noises I can try to help, just PM me or post here

https://www.clublexus.com/forums/gs-2nd-gen-1998-2005/910607-question-about-gs-is-300-exhaust-manifold.html

Glad you got it figured out brotha.

Just for other peoples knowledge, the early GS, 98 and 99, and maybe 2000 (someone may be able to verify this) only have two cats, (the manifold ones). On the early models, the mid-pipe has a resonator that looks like it could be a cat, but its just a straight through resonator.

My very first issue addressed when I picked up my 98 was clogged manifold cats. Running a full header, mid-pipe and cat-back now

Do you get better gas mileage with the header, midpipe and cat back ? I also have a 99 and think the cats are clogged to the point where you could hear them rattling and poor gas milage.

General past experiences are that a properly performing cat will offer no (or statistically insignificant) impact to performance.

You'd have to break the change to each individual issue. Was it the header, the midpipe, or fixing the cat (which can be a variable performance restriction), that change performance.

No, you're right. For a street car, the only reason to de-cat is being too cheap to replace it properly.

That, and we've had trouble finding an exhaust pipe without the cat already welded in. We can get a cat/flex-pipe for our Camry for $80, probably cost more to have one built without the cat.

I don't really do a lot of driving. I live 3 miles from work and rarely need to leave town. Maybe one 20 mile trip for different errands a week. I fuel up about once a month, so I really can't give good fuel economy figures. I did locate a used stock catted manifold first, installed it, fixed my issues and passed smog. After removing it for the headers, it has picked up noticeable performance, mostly high end. The car fells like it pulls hard for longer. Considering the weight reduction from a CAI and full exhaust with just two smaller mufflers and no cats/resonators, on top of better breathing/less restriction, and way lower under-hood temps going into the intake from there being no cats in the engine bay and my headers being wrapped, I don't think it has much of a choice but to be more efficient.

My reasons weren't really about fuel economy, more that it's cheap enough and easy enough of a mod to do that I felt it's worth while. I'm happy with the results and the way my car sounds. But I mostly am looking to save my known good cats for smog time and not burn them up by running on the car year round. I bought these cats from a guy used and have no idea how many miles are on them or how long they will last. So for me its good insurance to have them off the car and saved for California smog time.

You should have P0420 and or P0430 codes if you have bad cats. If those codes are present, you are likely right about the cats being clogged. Sometimes the codes don't come up though. Best way to figure it out is to use a vacuum gauge to the intake manifold and watch it while you snap the throttle. Vacuum gauges are very cheap and handy tools to have. And there is a plethora of information and pictures online about what the different readings mean that can explain it better than me.

The quick way is to remove your front O2 sensors and give it a drive. If it goes like a bat out of hell after that, the cats are restricted/clogged.

Yea, I did it because I'm in college without much money, but the smell is already getting to me so I may try to find one from a newer car at the scrap yard and weld it in my self

I need to check my gas mileage now, I know that before the cats were removed I would get 16mpg frequently and mines a 2005

sorry to tell u bubba but 98-05 is a 2nd genera GS300 and every single one of them HAS 3 catalytic converters, 2 on the manifold, the primary and secondary.. and a 3rd on the exhaust it’s literally a cat not a resonator. Go on rockauto.com or to any mechanic.. look up the cat for the 98-05 you’ll see the 2 for engine and 1 for exhaust lol. But hope you figure it all out bubba

https://charm.li/Lexus/1999/GS%20300...ter/Locations/

https://charm.li/Lexus/2005/GS%20300...ter/Locations/