I checked out this pad yesterday at REI and compared it to other "lightweight" options such as the Synmat UL 7. The Stratus' fabric is definitely beefier than the Synmat UL, more comparable with the Synmat Regular or the Downmat.
It's not box construction, but I heard an anecdote from a user while I was shopping: he said he liked the pad and that his down bag filled in the spots where it was welded-through, which enabled the down to loft in those spots. This would likely negate any insulative loss due to the welds. Your mileage may vary.
Myself, I bought the 40th Anniversary Prolite pad. Tested out the Prolite plus but the 2" of the 40th Anniversary was too good to turn down. Slept very well. Clocks in on my scale at 24 ounces (the Prolite Plus reg was about 23), R value of 4. Not really relevant to the conversation, but I went in wanting to buy the Stratus.