They exist for the purpose of determining what your air/fuel mixture should be. Whether you're at cruise or at WOT. Although the ECM is in open loop mode at WOT and doesn't use the 02 sensors, it does use information gathered during normal cruise to figure out how much fuel to use. This information is stored in your keep-alive memory in an adjustment table.
It's a common misconception that emissions related equipment interferes with power. Stock cats are about the only thing that this may be true about. Everything else either doesn't get used during WOT or is meant to keep your car in tune. It's a simple fact that a well-tuned vehicle is a low emissions vehicle. O2 sensors fit the latter category. You should keep them because your engine will run better, not because they keep emissions low.
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351W 89 Mustang GT Convertible