In the "old days," I used to see many Arthur Ashe Competition rackets by Head with the strings entirely pulled through the frame. This was because Head used Syntactic Foam to form the inner head area, and it would "melt" in high temperatures when the car was left in the sun (@ 170F or so).
Today's racquets are made of carbon fiber in an epoxy matrix. The bumper/grommets are made of nylon. There is nothing in today's racquet composition that should distort in the frame itself under this temperature.
However, strings usually will soften and loosen up under these conditions. Nylon usually melts at @ 400F, but it will loosen and go dead at exposure over room temperature.
I recommend not keeping your racquets in the trunk of your car. Even those thermos bags will not protect the racquets, strings, or balls when the temperature rises over 80F.
I also recommend the players use the external grip wraps, and change them often during the summer months. I see players all the time with grips that are too slippery and worn to allow players to hit proper shots.
It is pretty easy to take off the old grip wrap and rewrap the grip with a new one. (Pete Sampras, among others, would change the grip wraps during the changeovers between games).