I seem to have the same problem with my gsr swapped civic. But, I run consistently rich. You can even see it spray out of my exhaust during WOT. I would appreciate advise on mine as well as this problem has bothered me since I swapped the motor close to a year ago. I think I might know the problem, but not sure. I bought a "chipped" ecu off ebay. (Yea right). I think it just raised the fuel curve and just added more fuel. But, like you said, the ecu doesn't really get involved on WOT. Its up to the sensors. So, could one of my sensors be bad?
Another problem with my swap is I cannot cruise at a certain speed on the road. You know how you hold the pedal at a certain position and your car stays at say 45mph. Mine doesn't. It only lets me speed up or slow down. There seems to be no inbetween. I swapped out my civic throttle cable for a gsr one in hopes it would help. No luck. Please help!!!