1st off, you're right about spoons, they're treacherous!!!
Now, I didn't mean to imply that you or anybody else lies about their mileage, or maybe I did. I don't remember as my head was in the bottle.
What's true is your mileage has gotten PROGRESSIVELY worse. You've checked the basics: tire pressure, dragging brakes, tranny slippage, filters, etc.
Dirty injectors? I don't know, COULD be a possibility but at your mileage I wouldn't count on it. I had a Cummins w/350k miles on original injectors. I would've bet the farm that new injectors would give me better MPG's but such was not the case.
As for Stanadyne, they're a major rebuilder of injection pumps and also make their own additives which get pretty good reviews on the diesel scene. call a injector/pump repair facility and they probably carry the stuff (if you haven't looked 'em up on the 'net yet).
You mentioned something about dirty air filters, what setup are you running? and no, you may not notice adverse performance with bad injectors. How's your engine oil and what are you using, it seems that Pstrokes are "oil sensitive". Anyways, get the Stanadyne or maybe Lucas injector cleaner, fill your emptied filter housing with it, and see if that helps.
You mentioned "cleaning" your air filter, you're not using a K&N are you? Shame on you if you are.