All very good points.
RE weight: my xtherm came in overspec at 15.4 oz
RE packed size: They will probably be the same packed size. REI website states that x-therm in size regular is 4" x 9" and the Big Agnes QSL in Rectangle shape is 5"x10." We can expect the mummy shaped version to pack even smaller than that, so I wouldn't be surprised if they both packed the same. Looking at the end of the video clip at rei showing the packed rectangle QSL, it looks to be the same size as my packed x-therm: See http://www.rei.com/product/846673/big-agnes-q-core-sl-sleeping-pad#video-inner
RE R-value: The R-value is a bit more tricky. Like I said in my earlier post, I'm pretty hesitant about using these big agnes style pads as I've found them overrated in the past and subject to cold spots. I also think synthetic insulation will NOT withstand the abuse of constantly being rolled up and stored in a compact state. Similar offerings in the past have attested to this fact and have been verified by BPL. I agree the original neos were conservatively rated...but at the same time, who slept on theirs fully inflated anyways? Not only was it impossible to sleep on them in their fully inflated state, it also wasn't comfortable when the pad was rock solid. If anything, the neoair was accurately rated since it achieved it's stated r-value at a level of inflation that people actually used....good on them! Roger verifies this, stating; "But do note that there is little chance of exploiting the top end of the thickness scale shown above: you will compress the mat well below that when you lie on it, especially if you 'soften' it a bit. And that means the upper end of the measured R-value scale is similarly well out of reach. It just isn't going to happen."
If the QSL is accurately rated at r-value of 5, then the .7 extra from the x-therm doesn't make a difference to my own personal style of camping...anything r-value of around 5 or higher is okay in my book since I don't camp below 0F.
I guess one thing that we haven't discussed about too much, and that has me thinking, is the issue of thickness when the user is actually on it. Sure 3.5" looks incredibly awesome and is a whole 1" greater than the xtherm, but what does that mean in practice? As we all know, thickness on paper doesn't always equate to the same when you're lying on it. Current designs indicate that baffling/trestles are necessary to maintain thickness across the entire pad when the user concentrates his weight on a particular area of the pad (compare the old original neoair or old big agnes pads where one would sit on it, and their butt would touch touch the ground but the surrounding pad would puff up with the dispersed air vs the neoair all season, xlite or xtherm where because of the overzealous use of baffling, if you sat on the pad you were still supported). So what I'm getting at is that 3.5" means nothing if there isn't some internal design to hinder air circulation and support the user when they concentrate their weight on an area. Also, the thicker the mattress, the more issues you begin to run into. For example, if you use a shelter with steep sloping walls (think pyramid shelter like a Duomid, etc etc), that's less clearance for your head when you are lying down or sitting up , etc.
In practice, all I need is a couple mm's off the ground...enough where I can't feel the pebble or stick. That's fine in my book, and I really don't care if it took the pad 1" or 3.5" of inflated thickness to achieve that.