The fuel pressure regulator could indeed affect gas mileage greatly if worn and I suggest you check it out if you've tried all the usual things already.
As you look at the engine from the front, its hidden under the fuel rail and between the 2nd and 3rd inlet tract of the inlet manifold I think. It has a vacuum pipe sticking out there.

I know this coz i've just removed mine yesterday and fitted an external adjustable one. My 850 has been runnig rich ... 14mpg with a very sooty exhaust. I've changed everything ...o2 sensor, MAF sensor, temp sensor even exhaust and airbox flap is moving Ok and no vacuum leaks. My car runs on LPG as well so I've been using that but I would like it to run right on petrol as well. It was doing it before I had the LPG conversion. I dunno if its cured it yet coz I not been anywhere far, but it does feel good. Theres almost none of the strong exhaust fumes now and response is weaker when cold...before it was so rich it was very powerful from cold.