Today I started running a further downstream (about 4') O2 sensor in the GN (already converted to heated type), and I've noticed:

In cruise, the O2 output dropped about 30 millivolts. I've venture since it's heated and the cross counts were about in the same range, that backpressure was the deciding factor.
The GM ecms usually have a transportation time calc., for expecting a response from the O2 for some fueling changes, and on the 148 it must be kinda crude, since everything appears to be working just fine.