I was filling information on old ICs mostly from personal experience and back in X48/X58 days I wasn't much of a RAM expert. I had many kits with 1Gbit-D but not a single one, even the FlexII above would do 900MHz CL7 at any voltage, though I had multiple kits up to and above 1000MHz CL8/CL9 on X58 under 1.8V, hence why I thought these do not scale too well.
If you say so, I'm going to up it to ~2V for now. May be I will try to buy a kit in the future to get a better impression on the ICs