Actually I did run my previous Venice IHS-less (that's why I had to order a new retention bracket). I almost crushed my Opteron core because the retention bracket was uneven on the mb. I'm kinda afraid to retry it.
After that experience, I reinstalled the IHS on the Venice (a chip I know well, having it for months) to check what kind of temp difference I could expect with the reapplied IHS. I got good temps, 2-3 degrees higher than IHS-less at most.
I expected the same with the Opty. I guess I was wrong :(
My only hope is a great difference between retail cooler and XP-90, which I doubt.