People that "By pass cats" tend to have a piece of pipe made up that replaces the cat, i.e, take that cat off, put pipe in.
To have a permanant by pass pipe and valve would look messy and be awkward to fit.
There are some motorsport cats available that have much better flow than standard, would not reduce your bhp by much and probably (whilst more expensive than a standard one) be cheaper than adding pipes and valves round a standard one.
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