I recently added my 4870X2 to my loop and looks like i got the same "problem" like you, my MEMIO from one card is getting very hot (~80°C) under heavy load..
Have you finally found out how to fix this? I think my block makes good contact to everyhing on the card and i donīt want to mount it to hard since i already broke a memory chip on an old 1900xtx after iīve tighten the block to hard.
Was is just a bad contact or something else?
I did make a up on my first installation. The thermal pads have adhesive tape on both sides, which has to be removed(I'm right on that?). When I took the card apart I noticed that I'd accidentally left the tape on one of my pads. I can't be certain as I'm on a new card now (The 1st was for friend's build), but this may have had an impact on those temps.

You're right to not over tighten it.

MEMIO on mine is also VERY hot... 96c+ on stock air with a 50cfm 80mm throwing air at the intake fan as well.

I wonder if this is related to the "good" cards and "bad" cards (referring to test 2 and 3 in 3dmark2003)?
memio on my card is fine. However it plays up on those tests (It didn't use to). Only way around it on my card is gradual FSB increases while in windows.

RLM