Yes you can downclock with tighter timings just fine.
To get stable at 2000MHz on my Asus board, first things first, screw the XMP. Set it all manually.
, run it, and click on the SPD button at the bottom. Either print or copy down ALL of these settings. These are all the timings that your RAM needs to run at to become 2000MHz stable.
After that go into your BIOS. Set your RAM to 2000MHz (via multi, BCLK, whatever you need to do) and set 1.66V RAM and around 1.45-1.50V QPI/VTT just to start. Only reason I say 1.66V for RAM instead of 1.65V is my board will only let me choose 1.64V or 1.66V, no 1.65V to choose from. I think most Asus X58 boards are like this from what I've seen.
Then go to your RAM timings and manually set ALL TIMINGS YOU CAN. If the SPD that you printed calls for a looser timing than what is available, set it to the loosest timing. Remember, manually set whatever you can. Leave as little on AUTO as you possibly can, however do not move a setting from AUTO if you don't have a specific value to put in. Do not just guess.
Now you should be able to at least get some sort of stability. Run some stability tests and tweak your QPI/VTT voltage accordingly. You may be able to go lower, maybe have to go higher, you may have lucked out and it's stable already. It all depends on the IMC on your particular chip.
I could not even POST, let alone gain stability, at 2000MHz RAM on my board without going through this process. I went through two sets of RAM and hours of tweaking before I figured this out. Now that I manually set all of my timings, I still need 1.66V QPI/VTT (XMP said 1.70V at stock, so yeah BS) to keep it stable with HT on at 4.2GHz, but with the amount of vcore I need for 4.2GHz with HT on, I don't think I got too good of a batch. Your results may vary.
Also, when upping QPI/VTT this high watch your temps, especially with HT enabled. Even on water with 1.378V vcore, HT enabled, and 1.66V QPI/VTT things get pretty toasty in LinX. I'm not too worried about running the QPI/VTT this high. From what I've read, it's more important to keep your RAM voltage and QPI/VTT within 0.50V of each other. As long as you do that and have adequate cooling, you should be fine. Your other option is to reduce the memory multiplier and run at 1600MHz with tighter timings. With 1.325V QPI/VTT and 1.328V vcore, HT disabled, I can get to 1600MHz 6-8-6-24-1T 1.66V vRAM no problem. I could probably tighten up timings a bit more but I don't feel it's worth all the time stress testing just for that.Edited by c00lkatz - 8/4/10 at 11:05pm