The best replacement for high flow cat is test pipe.

The best alternatives to high flow cat are rated as following:
★★★★★  test pipe
★★★★  no cat
★★  resonator
★★  stock cat
★★  straight pipe
★  catless
★  test pipe evo x
★  resonated test pipe
  test pipe s2000
  cat delete
  test pipes

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Speaking about high flow cat, one probably shouldn't forget about flowmaster.

Other search terms related to high flow cat and their ratings are:
★★★★★ flowmaster
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high flow cat
test pipe
no cat