Pig rich on stock tune is a good thing for the K04 on pump gas. This is a deliberate engine development design and for a good reason. It provides cylinder cooling that reduces the risk of detonation and permits more aggressive timing advance to build power. Gasoline is excellent at absorbing heat. On the other hand, there has been shown to be little benefit on this engine to lean out the AFR's under high boost, high load, high rpm conditions and it is probably detrimental.
And yes, AFR's in the 11's and even down to the 10's at high rpm are normal on this engine on stock tune. Those AFR's also provide a measure of safety if the pump pressure should drop -- up to a point. The ECU is programmed to maintain target AFR at all cost to save the engine. So, what happens when pump pressure drops is that the ECU tells the fuel injectors to stay open longer on each cycle - that is called "widening injector pulse width." If pressure drops, the ECU compensates by increasing volume. This can only work up to a point. Once that adjustment range is exceeded, then AFR's lean out and the engine can get damaged.
Another downside to drop in fuel pressure is that high pressure provides excellent atomization of the fuel allowing it to burn more efficiently. Drop pressure and less atomization. Combine that with extra volume to compensate and you run the risk of another phenomenon that is not good for this engine -- the pooling of unburned fuel on the top of the pistions. This is an open invitation for engine failure and has been so documented graphically on another of the Mazda boards.
Bottom line, IMHO, is if you are going to increase exhaust side flow with a big downpipe, it is strongly advisable to upgrade the pump internals. It is very expensive to replace the engine on these cars.
