Most of this is old news to most of you, but there may be someone on here that is not aware....so here goes.
A turbo boost gauge is in my opinion a very important tool. Just watch the boost at cruise, and realize anytime your boost goes up, more fuel goes in and mpg goes down. Hold the rpm where the engine tends to not need as much boost. I suppose that's what the old timer's call the "sweet spot," because it is. That's where the torque is. Yes, you can listen to the turbo, but by the time you hear it spooling up, you're already dumping fuel. I prefer the pressure, as it's more accurate. If nothing else, the gauge will determine the sweet spot on your engine at a specific weight on flat ground. I drive by the rpm and the boost gauge on the highway instead of speedometer. My last truck was 68 mph, and I could run it up to 72 with a light load and still maintain mileage. I did test my truck at 62 mph at the request of the fuel manager, and it got worse mileage at 1400rpm because of the loss of torque. I also tried the same thing with a Detroit 12.7, and it didn't like anything below 1650 for mileage, as the torque would just fall off. I ran it at the ISX rpm and lost .5 mpg. So, with the little knowledge and experimentation I've done, I think the ISX likes 1550 rpm's, and the Detroit 60 likes 1650 rpm's. Firebird, what you said about the charge air cooler is a big variable in all of this, because I've seen pressure gauges that read normal, but the pressure never made it to the engine, and the fuel mileage and power was off considerably. Money well spent for that check.