tooter's exhaust experiment...

[xnamerxx]

I can't imagine their pulling the data from a front o2 sensor, or a thread in Wideband sensor. Most dyno shops use a wire sensor that sticks into the back of the tailpipe.

Given the rest of the engine is fairly stock, I'm gonna go with there was an exhaust leak at the axle back joint. Exhaust leaks don't hurt performance by affecting AFR they hurt performance because an exhaust leak creates turbulent air flow which restricts flow in the tube. Even small exhaust leaks can cause huge losses of power.