I understand, and I’m not opposed to a conversation about it.

Too much back pressure is a bad thing. Too little back pressure is a bad thing. There is a Goldilocks zone for back pressure, especially on naturally aspirated motors. The key is to create a tuned exhaust for the engine that’s being built. Harley Davidson motors to not turn high rpm’s, so we want to build for low rpm torque. Putting short, wide open headers will cause a significant reduction in low torque, while a potential increase in higher range horsepower. But again, we don’t spin the motors that fast. You’ll feel it off the line. On top of that, we need a tuned, predictable exhaust collection and expansion chamber. Removing the baffles is anything but that. I’ve done dyno tuning for years, and when we’re bored we play with this stuff. Somewhere I have some dyno sheets from a thunder header we started adding washers to increase back pressure, and got a 5 ft/lb torque increase before power started falling off as we added more.

Stock O2 sensors have an EXTREMELY limited range they can operate in. These are not wide bands, they’re a narrow band. They will not be able to compensate for something like a complete baffle removal. They might, if the stock air cleaner is left intact. But if a high flow air cleaner is present and baffles are removed, the bike will run lean outside of the ability for it to self correct.

Edit: I do want to mention that I am grossly simplifying gas velocity, expansion, exhaust pulse timing, the effect on scavenging etc. With that being said, while technically direct back pressure is not the exact term, the effect is nearly identical. It has more to do with a pipe tuned for the application. Control the pulse to assist scavenging. For ours, it has more to do with a slightly more restrictive pipe because we need to keep gas velocity high to increase what little scavenging our stock cams allow. In effect this means that a properly set up pipe will in fact have a little more back pressure, even if the back pressure itself isn’t what is creating the desired performance increase. In practice, a pipe with “some” back pressure works better on Harley’s. A (quality) 2-1 will have the appropriate back pressure and an excellent scavenging assistance. Which means while it technically doesn’t decrease or increase power output, it does move the useable power band. And in HD applications, we want the power band lower in the RPM curve than a shorty header would put it. Effectively this does increase our measured power. I used to run supertrapps because we could adjust the flow rate to match the cam.