I don't know about fuel mileage, but look through the front grilles and check if your intercooler is ballooning. All of the horizontal tubes should be the exact same height.
Mine aren't (pic attached) and I have noticed the same boost numbers as you. About a year ago it was 19 to 21, and now it's only 17 or 18. I also threw a CEL for some code I don't remember and was limited to 10psi until I cleared it. Once I have the money and time to install a better aftermarket intercooler I'm hopeful it'll go back to 20 and the random CEL will stop, as confirmed by a few older threads I found a while back.
2007 Solstice GXP
#3 ASP, DET and NWOR SCCA
"It's a Kappa - not a classic"