It's sounding like a fuel pump. Have you got a fuel pressure reading at WOT with a load?

To check the cat hook a vacuum gauge up somewhere on the intake manifold. Start the engine and let it warm up to normal temp. With the vehicle parked record the vacuum at idle. Then quickly press the throttle to wide open and then quickly snap it shut. Do this three times and record the vacuum at each wipe open moment. Do it a fourth time and record this vacuum reading. If the forth reading is more then one in-Hg lower than the reading at idle then this could indicate a restricted exhaust (like a cat but could be a muffler). At this point I would pull the cat and inspect it.