I am no engineer but I do not see how back pressure can be better. Less pressure= more exhaust gasses can get out faster=emptier cylinder=more space for fresh air+fuel= more power. Correctly designed exhaust manifolds can even make negative pressure and suck exhaust gasses out. I can see how bigger pipe or removing cat can slow down gasses because of greater cross-section area reducing the suction effect and there-for evacuation of exhaust gasses.
Now 2 stroke engines are completely different case but that's another story.
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