Rear o2 does NOTHING for fueling aside from some cars doing a weird cat test thing during cruise.

No "back pressure" is retarded... I ran my fieros with a 3 in tube going from the manifolds straight down about 7 inches worth of 3 inch tube..

The fact you are making "half boost" is not related to exhaust. Any exhaust restrictions are going to increase boost, anything else is not going to affect it.