Ive noticed a little drop in economy at cruise since I got a chip that increased boost. With my hypertech chip I would have like 2-3psi at 65-70mph, and then I put in a stock chip from a 95 K2500 L65, and boost in all cases went up, as well as power, but my 55-70MPH mileage seemed to suffer 1-2MPG when boost was at like 5psi. my max fuel rate with the stock L65 chip was lower than the Hypertech chip, but I still had more power. It could have been the chip telling the IP to dump more fuel too, I dont know. I havent done much with my new Engh Chip to know yet, but boost pressure is a little lower at cruise.
Although I still tracked a little over 18MPG combined with that stock L65 chip, and thats driving 75-80MPH on the interstate, spirited stop and go in town, and some 45-55 to and from work each day. (my other car is a Z28, so its very hard for me not to want to get up and go from a stop)
Im also surprised the ECM doesnt have programming to defuel if it doesnt detect the increased boost it asked the turbo to send. I understood its fuel dependent, but figured there is some feedback, since the ECM's intent is to send more air to increase the air/fuel ratio. Maybe I can design that for myself

since all the sensors are there.
I'm also a little fuzzy on how the ATT can flow more air at lower PSI when volume of the intake is constant. Or does it just provide sufficient, with a more efficient turbine on the exhaust side that lets more free flow out reducing backpressure?