The secondary air injection pump has nothing to do with how the car runs. Its soul used is to heat the catalyst during cold start so they are ready quicker to reduce emissions. As far as poor performance, how is fuel pressure/pump draw? injector spray? engine harness? If you don't have to do a tailpipe sniff test, throw that pump in the woods and get the shorter belt. Don't spend money unless you need to, the MAF is an expensive guess, also you can take a razor and slice the throttle valve actuator's shrink wrapped harness and look at the internal harness's condition, the short out and take out the EA module on same vintage v12s.