Increase to 15psi max, and control it with an mbc. I think this is how people did it back in the days. Even though you add more boost, there is no mapping for it. The ECM is still requesting stock boost and you are throwing more air at it, which in turn makes the car run more lean. The ECU can compensate but for so much by adding more fuel if you have a wideband primary o2 sensor, but that does not account for ignition timing or retard timing as a by product.