For our temperature testing, we measure the peak memory temperature under load. A reading under load comes from running Cyberpunk 2077 for 30 minutes.
GDDR6X memory temperatures are also kept well in check. I wasn't sure if the 21Gbps modules would run noticeably hotter, but at least with these two cards, memory thermals are absolutely fine. The RTX 3080 Ti Founders Edition GDDR6X runs well into the 90C range and that's slower memory to begin with, so we can't complain about these two 3090 Tis, though performance may vary depending on the specific model you are looking at.