It was going to be difficult to test on those rigs because those are my "special" rigs - one is my personal desktop PC and the other is the PC connected to TV. I have 4 long USB lines coming out of the PC case for 4 additional GPUs. So lots of things are installed on those machines consuming more RAM.
In your case though, did you confirm that you are getting higher hashrates with 16GB?
Also, sounds like at least you achieved stable 620 hashrate on 8GB. Correct?
I went from like 400 per card at 4gb to 620 with 8gb I haven't gone to 16gb but 100% would achieve a higher hashrate