Don't worry about the air pump - once you're warmed up and the system is in "closed loop" the pump is completely diverted, for good reason: any extra oxygen would send a lean signal to the ECM and result in an over-rich injector command. The pump is only injecting when the engine is warming up. The non-heated O2 sensors located after the two exhausts collect has always been a bit of a problem, they run so cold that they often quit working when the engine idles for some time (like at a long stop light) and you're left with fixed commands (i.e. open loop and no fuel trim adjust) until the hot exhaust during acceleration heats it up and gets it moving again. Locating the sensor in an "H" pipe would not only make the sensor run cooler still, the whole system works better and responds to the constant fuel trim adjustments (up to maybe 16 times/second) faster if the sensor is kept hot and in a strong exhaust stream. This is why on in-line engines it's right on the manifold at the earliest collection point, not the pipe.