How about taking it down a notch?

So here's the deal. The S300 doesn't have anything to do with the fuel pressure. When you have the tuning done, the injector mapping will be done at the boost levels you want to use, so you have safe fueling at that boost level. Generally, fuel pressure regulators are already referenced to manifold pressure, so they will increase fuel pressure to compensate for boost. The injector mapping is then done to add enough fuel for the extra power under boost.

A wideband isn't needed for anything but tuning.