Hi All i have a lil bit of a power loss problem.

I have just put a 3" straight through stainless steel exhaust on my MY93 Wrx import, I have taken off the old down pipe which had a sensor attached but looks to small to fit any of the sensor holes on the new down pipe so i have put a bung in the top of the down pipe, Now the car when pulling hard seem to be hesitant and then starts to pull ( i could only just gain on a standard RX8 the other day does anyone have any advise on it and would a remap sort it out even without the o2 sensors?

Thanks
Gaz