These JRB products are great, and deserve their popularity. But I am still looking forward to a long term test of the HHSS. Tom Hennessy, for what it's worth, is still very pro this design and has not yet gone to another. I have used it quite successfully myself. Although, for me, their was a bit of a learning curve to get it below the low 40's or maybe high 30's. But the addition of the kidney/torso pads gives a big boost. It can be boosted significantly with the "Garlington insulator", a trash bag with a space blanket in it. And the original design of using clothing that you are not wearing to bed ( IF there is any not being used )can, again, really improve the warmth. And of course, the system includes some built in moisture/wind protection with the silnylon undercover. I have the larger, heavier Explorer model with the 35" wide pad. The listed weights are: silnylon undercover= 8.75 oz, underpad = 11 oz., for 19.75 oz total. ( the standard for the UL Backpacker is, I think, only 13 oz total).
Though I suspect the down under quilts will be a bit warmer for about the same weight, this is probabbly still a good alternative. Considering that the "weather-shield" is already part of the system, and cost. There MIGHT also be an additional wet weather advantage to the open cell foam compared to down. And there is a cost advantage: $129 size reg. Naturally, all of that nice down cost's more.
But if using the No-Sniv as an underquilt, you would have the multi use advantage of wearing it for camp warmth. That's a big advantage, but it would already be taken care of if using the No Sniv for the top quilt.
So I hope there is a thorough testing here someday! Even though maybe not quite as warm, it has other factors that make it a possible alternative.