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.
can you please update the photo to point to where you think there is a problem? I am not seeing a difference...
Sir William's Fleet
Mean 2007 NA 2007 Solstice ordered on 8/24/06 - Picked up on 12/12/06 - Totaled on 6/12/10
Mean 2008 GXP Acquired 6/21/10 with 1275 miles on it - Summer Solstice !
Deep 2009 NA Coupe Acquired 11/2/13 with 1040 miles on it
Mean 2009 GXP Coupe Acquired 8/27/16 with 9100 miles on it