Hi.
Welcome to our Mattress Forum! 
I am glad that you find our site useful.
The thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful because whether a thicker or thinner mattress would be better or worse for any particular person will depend on the specifics of the materials (type, firmness, etc.) and on all the other layers in the mattress. Thickness is only one of many specs that are used to make different mattresses that perform and feel differently and that makes a mattress suitable for one person and not another. There is more about the effect of thickness in post #14 here.
Thinner latex mattresses are common in many parts of the world but only your personal testing can tell if this would be an appropriate choice for you. A 6" mattress can be adequate for adults as well and regardless of how thick or thin a mattress may be … the most important part of the “value” of a mattress is how suitable it is “as a whole” for your particular body type, sleeping positions, and sensitivities in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
Arpico is a foam supplier in Sri Lank producing high quality natural and organic Dunlop latex, many of our members here are sourcing their foams from Arpico. They are actually mentioned quite often both here on the site and worldwide here
Weather 100% natural Dunlop or their organic Dunlop Arpico’s foams are certainly high quality and very durable so if any of the mattresses you are considering are a good match in terms of PPP and there are no lower quality materials or weak links that would compromise durability and based on all the other parts of your personal value equation that are important to you they would certainly be well worth considering.
I can’t speak to why North Shore Linens would have these prices, but they have a history of reasonable prices that some consumers here commented on. Once you reach their showroom make sure to look at the product and determine the quality of the overall product, including their organic cotton cover, return policies and so on.
The most reliable and “accurate” method to see if a mattress is suitable for both comfort and support is generally your own careful and objective personal testing on the specific mattress you are considering as a purchase. In the showroom testing and depending upon the variables listed previously when lying on your side I would pay attention to the comfort and pressure points to decide if you have enough pressure relief or if you might need extra 2"-3" of latex (which is offered with their 8" Deluxe Mattress)
We are working on the back end to implement and finalize the new design element – Talk to the Experts and while it is under construction, we have eliminated “New Topics” for Phoenix so we can effectively service consumer members and manage the increased workload demand. However, the “Ask an Expert” tab of the site is taking new topics for any of the Experts listed.
I hope this information is helpful and I’ll be interested to learn of your eventual decision.
Phoenix