The best replacement for test pipe is high flow cat.

The best alternatives to test pipe are rated as following:
★★★★★  high flow cat
★★★★★  catalytic converter
★★★★  straight pipe
★★★  downpipe
★★  gutted cat
★★  resonator
★★  race pipe
★  hfc evo x
★  hollow cat
★  hi flow cat
★  high flow
★  hfc
  non resonated test pipe
  down pipe
  resonated test pipe
  stollen test pipe

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Speaking about test pipe, one probably shouldn't forget about stock exhaust.

Other search terms related to test pipe and their ratings are:
★★★★★ stock exhaust
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test pipe
high flow cat
catalytic converter