I've pretty much got the same problem, and absolutely horrible mileage right now. It's not that it gradually went down or anything like that, but just one day i started to lose about 80-100Km per tank. Note that couple of months before I also replaced the O2 sensor and full tune-up. Going about 110 on the highway (same as about 65 mph) feels absolutely horrible and takes a HUGE amount of power. Don't even want to mention if someone else is driving in the car with me, makes it even worse.
I had tramission problems while the mileage went down too, but have since swapped out my faulty transmission with a rebuilt unit and it's working great. No change on the mileage after.
I also took it to the mechanics, and they checked everything over - cleaned the MAS, checked plugs, timing, rotor, cap, timing, error codes, i got a K&N Filter. Nothing wrong.
They did hook it up to check the exhaust and did tell me that i have a leak in my EGR valve - so of course I asked them to fix it.
Still no better mileage.
I also got the fuel injectors cleaned, and whatever other bs they charge you a huge amount of money for.
Still no better mileage.
One main question that I have is about the O2 Sensor. I wanted to see if there was a difference with it plugged out. I unplugged it, drove around for a tank, and completely no change.
But how come the check engine light didn't come on?
I'm gonna try to do the error code check on the computer and see if anything comes up.But the mechanics said they already did this, and no errors coming up.
Could this also be the Cat? Hate wasting money on this, and don't want to ask/trust the mechanics again.
(For the hell of it while i was in Vancouver over the weekend, I stopped off at the Nissan dealer to ask how much is a cat and muffler...they quoted me OVER $800!!! JUST FOR PARTS!! I started to laugh.)