So I looked at the only other stock run of PR I have a screenshot of with GPU temps fully listed.
Pre-heatsinks & vertical mounting installation, Peak Temps during Port Royal were (Don't have GPU main temp for some reason):
GPU 2 hit 52C, Mem1 50C - Mem2 54C - Mem3 48C, PWR1 48C, PWR2 41C, PWR3 46C, PWR 4 32C, PWR5 34C
Post heatsinks on the back (which are not over VRM areas at all):
GPU 2 hit 50.9C, Mem1 44.0C - Mem2 50.2C - Mem3 47.0C, PWR1 47.5C, PWR2 40.2C, PWR3 45.8C, PWR 4 31.9C, PWR5 32.5C
If that sort of effect on memory temps holds, that's pretty impressive for some simple rinky-dink aluminum heatsinks being thermal-taped to the back of the GPU. The only other factor could've been ambient temperature, but I'm fairly certain the previous stock run was done at 70F ambient, the one I ran today after the installs was run at ~71F.
Makes me think Mem1 is on the back of the card, because that's where I tried to get the heatsinks as close to as possible, over the back of the GPU core, and over around-about where the vRAM chips are on the top of the card.