Hi UnderSlept,
Welcome to the Mattress Forum! … and it’s great to see that you’ve already read the tutorials and clearly have some good basic knowledge about the quality and durability of mattress materials 
You can see some comments about the Luxisleep mattress in post #10 here.
As you probably know the SBT layer acts in a similar way to buckling column gel and will “buckle” under areas of higher weight and pressure (such as the hips/pelvis or the shoulders) to help redistribute the pressure and relieve pressure points but will stay firmer and more “supportive” under the the more recessed parts or the body (such as the small of the back or the waist) to help maintain good alignment.
When you sleep on a mattress the upper layers of foam will compress and deflect more than the deeper layers or components partly because the comfort layers are usually made to be softer than the deeper transition and support layers or components of a mattress (and firmness/softness is also a factor in the durability of a material) and partly because they are closer to the sleeping surface and subject to direct compression without any layers above them absorbing some of the compression forces first. It’s this constant deflection of the materials in the upper layers of the mattress that softens and breaks down the foam materials and leads to the loss of comfort and support of the mattress over time.
What this means is that the SBT layer would be most subject to foam softening and breakdown if it was used as the top layer of the mattress and if you flip the top layer then the foam layers above the SBT layer would absorb some of the compression forces that are the result of sleeping on the mattress before they reached the SBT layer so the SBT layer would actually be more durable in this configuration than it would be if it was on top of the mattress. Either way … there are no lower quality materials or weak links in the mattress that would compromise the durability or useful life of the mattress relative to your weight range even with the SBT layer on top.
You are the first on on the forum to mention them so it gives me a chance to make some comments about their mattress so that others that may be considering them can read them as well. Thank you
The OSO mattress is also somewhat unique because it is a two zone design which allows you to have a softer zone under your shoulders. This can be beneficial for side sleepers especially because the shoulders are generally wider and lighter than the hips and having a softer layer under the shoulders allows the shoulders to sink in more deeply which can improve pressure relief under the shoulders and also can also improve spinal alignment in the upper body if a firmer layer would raise the shoulders higher than they should be. There is more about the pros and cons of zoning in this article and in post #11 here and the posts it links to.
Having a softer zone under the shoulders would be the most common configuration for the mattress but for those that either need or prefer more firmness under the shoulders and upper body then you can also rotate the mattress 180 degrees and then the softer zone would fall underneath the lower legs and calf where it will have no effect on the shoulders or upper body alignment and with the exception of having slightly improved pressure relief under the calves and lower legs (which for most people wouldn’t make much difference) it would be similar to a mattress that uses the same layers that doesn’t have any zoning.
The materials and components in the Ososleep mattress are as follows …
Zip knit cover that can be removed and dry cleaned
1.5" of blended Talalay latex
1.5" of 2.0 lb polyfoam
6" of 2 lb polyfoam
The Dreamcell cylinders are made from 70% natural/30% synthetic Dunlop latex blend and are 1/2 the 6" height of the Dreamcells that are used in the Reverie Dream sleep systems but they can’t be accessed and rearranged like the Dreamcells in the Reverie Sleep system. They are placed inside a cutout in the 6" base layer.
All of these are good quality and durable materials and there are no lower quality materials or weak links that would compromise the durability or useful life of the mattress relative to your weight range (or any weight range).
Phoenix
PS: Reverie which is Ososleep’s parent company is a member of this site and I have also talked with Ososleep about membership and they will likely become a member as well in the next week or two. (NOTE ADDED: they are now a member of the site).
Phoenix