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Certainly buying catless uppers and leaving stock secondaries is the cheapest way to improve flow. Stock secondaries flow a few percent more than stock primaries, but having only 1 instead of 2 helps.
I understand that quality high flow cats like 200 cell HJS can work with a stage 2 tune, but I am not sure about higher cell or lower quality cats or about just running stock primaries or stock secondaries. The less backpressure the better when pushing the limits.
I have run catless on a number of cars and never complained about smell except when I did not have a catless tune. I don’t spend a lot of time idling in stopped traffic in stagnant air, however. That is the only condition under which I have noticed a smell.
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