Unfortunately test pipes will cause a hissing noise unless you have resonated high flow cats or resonated test pipes. However even with resonated test pipes it will reduce the hissing noise, it is resonated high flow cats that ultimately quiet down the hissing the most.
As for the O2 sensor you can fix that with a O2 bung extension to keep the sensor away from feeling the immense heat that the test pipe produces.
My best suggestion would be to look into resonated high flow cats
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