I am battling my 1999 Impreza Outback Sport's poor running issue right now.  It had started running poorly at high rpms 4k+ a month or so ago then this week as I was driving home i had felt it sputter a few times at a steady cruising speed. Died at idle. Let sit for a day started and BARELY ran at idle but would run mid-throttle range and got me most of the way home. Same 0440 and 0420 codes have been showing for almost a year. Check fuel pressure before and after the under hood filter, nothing, doesn't even flicker. Fuel pump turned on and 10v + at the fuel pump according to my service manual. Replace pump, same thing. BARELY runs at idle and takes throttle ok until spark plugs foul out, BLACK and fuel soaked. 0 zero fuel pressure... pinch the line before the fuel rail when testing and still zero. Disconnect the line and when turning the key on it will fill a beaker about 250l when it primes for 2 seconds. I've found newer style pump housings crack but mine is the old style where the pump pumps straight up the fuel line. 

Have you tested the fuel pressure regulator?

 

How many miles on this?

 

Usually a fuel delivery problem is the engine will idle but runs like crap at higher RPMs as the fuel system can't keep up with demand.

 

Have you looked at the spark plugs to see how bad they are?

 

When was the last time this had a tuneup and a timing belt done?

Did you check voltage at the pump while it was running?

 

Did you heck the connector terminals for burning/corrosion?

 

Can you get another pressure guage to try?

www.brighterideas.com

 

Use a LoadPro to make sure your meter is not telling you you have voltage under no load.  Loaded, voltage falls on its tail.