First, there is absolutely ZERO reference for fuel pressure on the stock TBI unit. You don't get a reference simply by placing the regulator "close" to the pod. Factory TBI uses a fixed pressure...
Hmm... Neukam is spot on. The TBI FPR is a typical diaphragm plus spring type regulator. Fuel is exposed to one side of the diaphragm, the other side, with the spring, is exposed to atmospheric pressure through a hole in the regulator hat. Because of this port, changes in atmospheric pressure will change the fuel pressure. The physical placement of injectors and regulator atop the throttle body and inside the air cleaner ensure the pressure at the bottom of the injector and pressure at the atmosphere port of the regulator remain the same relatively speaking. This is well known science.

I can show you lots of articles that support the FPR using manifold vacuum as the reference. Even the EBL tech articles discuss doing it this way.
Understand this is not the OEM method, it's not an "improved" method, and it's not necessary for a stock or mildly modified vehicle. This is a technique designed to allow one to obtain maximum fuel delivery under load while reducing fuel delivery at low load. It reduces the total range of VE values across the fueling tables and can reduce the work required to obtain good driveability. This technique is generally used for higher HP vehicles and its a workaround for the limits of the OE ecm and TBI injectors.

As for the 3 wire O2...the factory O2 on this vehicle is a single wire. A 3 wire simply adds a heating circuit, so the other 2 wires go to a power and ground. A 4 wire sensor will have the dedicated ground for the signal...3 wire does not have a signal reference ground...
I haven't talked with Neukam much but I have to wonder if the poster spent time at a GM facility or a GM dealership in the '90s. We are hearing of things that could be considered arcane knowledge today. The one wire O2 is really a two wire O2 without the second wire. The "wire" for the signal return is found in the body of the sensor. The sensor is attached to the exhaust pipes which provide a great path for ground. And the exhaust pipes are connected to the block. But the exhaust pipes and engine are hung in rubber which provides a poor signal path to the body. Engineers connected the signal return from the ecm to the rear of the engine block or cylinder head, close to the exhaust pipe without connecting directly to the pipe. Neukam is suggesting there's a possibility that ground is bad which would cause a false lean condition. Neukam is further suggesting the rare but real possibility that the ground for the heater has been connected to the same point as the ecm O2 return wire, and that the a bad connection to the block combined with heater power unable to find ground could add to the issue.