Thomas, i too am a wide-body, albeit a very short wide-body. i can use a WM Highlite - short length, which is a very fine bag (both in quality of construction and performance), but can barely move inside, making zipping/unzipping downright difficult. The tight lower body is fine, it's just that, IIRC, 59" shoulder girth. Also, WM is generally spot-on in their claimed lower temp ratings, and i consider myself a cold sleeper (though i need non-hi-loft layers to get to the Mfr. claimed lower limit of range).
i prefer the Montbell SuperStretch bags over my WM bag. However, to get equivalent warmth in an MB vs. WM bag, the MB will weigh more, IMHO.
The MB SS system will avoid the cold spots that might be caused by larger air volumes next to your body. It also allows ease of movement inside the bag while excluding the extra air next to your body, even at your feet - sort of a "win-win" situation.
Figure a MB SSDH #3 will be 23oz with 0.5oz more down (lower fill power, but let's not get into that issue here) as cp. to the WM Summerlite.
The SS system allows shoulder girth ranges, IIRC, from ~53" to ~71" with its GENTLE, non-compressive SS system. It's very easy to layer a hi-loft jacket (and pants, if so desired) to extend the range of the bag w/o compressing either the hi-loft clothing or the bag's insulation. Under these circumstances you might be able to take a slightly heavier (than its WM counterpart) down further than you would with the tighter fit of WM. Though as Shawn Basil has recently pointed out in another Thread, he achieves better results by laying his hi-loft clothing on top of his bag to extend the range, thereby eliminating compression and loss of loft.
The SS system avoids the issues some people have of a roomier cut WM bag and heating that extra air, particularly at the footend.
MB prices are comparable to WM prices.
Just a thought.