I was out in a PTJ today with a short-sleeved Merino (non-Rapha, Oh the shame) baselayer under it. Definitely not boiling, despite the liberal amounts of sunshine and allegedly 13degC. Admittedly, it's possible the biting cold 20mph winds may have had something to do with that.
PTJs and all their iterations are for 10 degrees and lower. I boil when I wear it in anything above 10