Is there a 180HP conversion available for a 2011 160HP Cessna 172R with a Garmin G1000?
Not a solution, but I looked at this a couple years ago.  I have a 2005 172R G1000 in a flight school and students tend to go to the 180hp planes, so I looked into the upgrade.  I did find that Cessna did a STC for a limited, sequential group of Serial Number 172R G1000's.  I believe they might have been done for Embry Riddle, but not positive.  I actually found this out from FAA when I was calling them about a STC or Field upgrade.  Got to the same point as your e-mail string.
Bottom line - it has been done.  It can be done.
When I called Cessna about the details they did tell me the block of SN's it applied to, but after that they would not share anything.
After looking into this again today, talking to Air Planes (they are very nice, and they would love to be able to sell a kit for this!) and remembering my conversations - I think I'll do some digging again.  I'll post if I find anything
Mark - 191CA