Got a PM on this lately, here's the rest of the story:
- It was the sender, as verified by a Zeitronix sender hooked up to my ZT2. Oil pressure was actually fine.
- The Prosport sender was also the source of my oil leak. It was broken to the point of leaking. It wasn't leaking at the threads... it was leaking through a seam in the sender. The Zeitronix sender was also significantly easier to install, since the sender body was smaller than the 22mm hex, I could just fit a deep 22mm socket over it and wrench it in rather than dealing with crow foot attachments at odd angles.
- The leak got oil everywhere, including the passenger side CV boot... which softened it enough that it split... and I couldn't get the shaft out of the hub, so I replaced the entire corner of the driveline.
All told, that Prosport sender cost me nearly $1000 once I got all it's damage cleaned up. More if you count the oil pan conversion, but I have a Perrin header now, so that could be considered part of the header install since it doesn't fit with the 02 WRX pan.