With a global mean temperature increase of 1 C since 1970 there was an increase in peak temperatures (avg) of about 2 C. So till 2050 it will be somewhere around 3 C for the average peak in summer. If we look at the ramp up since 2008 we can expect more like 5-6 C higher temperature records than today. So in the 2050s there will be some summers with 45 C records and the average hottest day every year around 35-37 C
Edit: and not to forget that this is only talking about how high the peaks every year are. The length of heatwaves will also increase by a few days. So where it was maybe 32 for three days and then 35 for one day, followed by a cooling thunderstorm it will be more like five days of 35 followed by a day of 37 and then a much more intense thunderstorm than what we know today.