well I am no expert myself, but basically the flexi pipe is part of the exhaust system at the manifold end, the factory fit is extremely restrictive, replacing it will free up and let it rev more freely.
You need to consider if the k&n and flexi are the only mods you want, as if you think you will want to change the entire exhaust over time, you may be better off with a manifold - many of them include the flexi part.
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