Muffler cut

Don't cut off the catalytic converters. They will muffle the sound a little, but you need the oxygen sensor in front and behind them. The sensors behind the converters are to monitor the converter's efficiency, not for flow, but for cleaning the exhaust. The converters for the last many years have been very free flowing so you won't gain anything by removing them. What WILL happen if the Engine Computer doesn't see their signals is it will turn on the Check Engine light and set a fault code. If you choose to drive it with that light on all the time, you'll never know when a different problem is detected. Also, there is a set of conditions that must be met for any fault code to set. One of those conditions is that certain other codes are not already in memory. That means you could have a sensor failure that you can't feel but it could cut down on horsepower or throttle response and there won't be a code or any way for you to know about it.

Don't gut the converters either. The front oxygen sensor switches between rich and lean many times per second. If the converter is working properly, the second, (downstream) sensor will switch very slowly, maybe once or twice per minute. That's what keeps the Engine Computer happy. When the converter loses its efficiency, such as when someone guts it out, no change takes place in the composition of the exhaust gas so the downstream sensor switches at the same rate as the front one. That's how the computer knows the converter isn't doing anything and will set a code and turn on the light.

What country are you in? You typed more accurately and clearly than a lot of people do.

No. That's an excuse used by people who don't know the correct reason for burned valves. Racing engines want as little back pressure as possible and they don't suffer burned valves. Also, if less back pressure is bad, more back pressure must be good, right? Tell your friends you're going to give them an advantage by squeezing their exhaust pipes closed! That will give them all kinds of back pressure and you can leave them in the dust!

Seriously, out of the four strokes, the piston only develops power on one of them, the power stroke. Energy is USED by that piston on the other three strokes and you would like to keep that needed energy to a minimum. On the exhaust stroke, energy is used to push the gases out. The easier that can be done, the more power can be used to turn the wheels.