You have a point there. It could be running lean with gutted cats but not for the reason you're thinking. At WOT most engine management switch to open loop operation (they ignore the O2 sensor) and use a fuel map that should normally be on the rich side. They sacrifice some performance to prevent detonation and save the motor. If you've opened up the exhaust and the intake, your motor may be moving more are than the ECU can compensate for. You can try larger injectors or tweak the fuel pressure up a few PSI so you get more squirt at WOT. Just my $.02.