At the start of each test I let the MBP run down to 80% charge and then cool down for 5 minutes in the outside 18C temperature to bring it down to around 28C chassis operating temperature. Then I plugged the USB-C charge cable in and let it charge back up to 85% charge while monitoring its temperature.
Here are the results of temperature readings at the 85% charge level. I monitored these further and they didn't fluctuate much from this so I think that is a good representative temperature for comparison purposes.
Left-hand port charging was considerably hotter internally (54C vs 30C) than right-hand port charging. Current draw was around the same in both cases, certainly not enough to affect this huge temperature difference. That leads me to think that I either have a defective left-hand port charge circuit or the circuitry on that side is less efficient.
I also measured chassis temperature (with a laser thermometer) above the touch bar - left, centre and right. Since I'm in Australia I thought it would be fun to use the same temperature scale as used by the BOM to illustrate the differences in temperature...
The chassis temperature differed by only 6C but it was still quite a noticeable difference.
These tests were done while the MBP was idle. When I was doing some work and charging via the left-hand port I noticed that the fans would go into overdrive, while right-hand charging didn't have the same effect. This is unfortunate because it means I have to have the power cable across my mousemat whenever I'm working on the road if I want to avoid the Mac heating up so much.