Hi TDI Hoo,
The mattresses at CozyPure are all latex mattresses which are a “fast response” material and the mattresses you have been testing are all memory foam mattresses which are a “slow response” material. Cheryl would probably “shudder” if someone was comparing her mattresses to a memory foam mattress 
While latex can be as pressure relieving as memory foam in a comfort layer (and their mattresses come in a range of softness and firmness levels) and a latex support core is higher quality and more adaptive and supportive than the polyfoam support core in either the iComfort (which is low quality 1.5 lb polyfoam) or Tempurpedic (which is higher quality 2.2 lb polyfoam) … they are very different in terms of feel and performance. Comparing latex with memory foam is really an apples to oranges comparison.
A latex mattress generally needs a firm rigid and non flexing foundation with either with slats that are no more than 3" apart (and preferably less) or a wire grid non flexing surface. If your foundation is one of these types and is still in good condition then it should be fine. If you have a flexing boxspring … then I would recommend that you change it. While a box spring can work with a latex mattress (as long as it is still in excellent condition with no weak or soft areas) … it will change the feel of a latex mattress which is usually designed to work best with a rigid base. There is more about different types of foundation and how they match different types of mattresses in post #2 here.
Winndom is a good local manufacturer IMO and tends to use higher quality materials than larger manufacturers in each price range and they are very responsive to their retailers but any mattress no matter who makes it is only as good as the materials in it so it would be important to make sure you knew all the quality specs of the layers of any mattress you were considering (and if the retailer didn’t have these they should phone Winndom for you to find out) so you could make meaningful comparisons in terms of quality and value with other mattresses.
The OMF uses blended Talalay latex and is a good quality and value mattress for those that it “fits” their needs and preferences (they only have two models which either work or don’t). They don’t have the same options as CozyPure to customize the mattress to your needs and preferences and the ticking and quilting is lower quality as well. The blended talalay is also not as “natural” as the Dunlop for those where natural materials is more important but both are high quality materials. In other words … the Cozy Pure has more options and is a higher quality more premium mattress but also carries a higher price.
King Koil is now a brand and the company itself is called Comfort Solutions. They are one of the largest brands (top 5) and tend to be included in my recommendations to avoid major brands but if you can find out the layering of a particular mattress they make (and they are not usually open about this) and it is better value than a similar mattress … then it would certainly be worth considering. They do make a series of mattresses called XL (Extended Life) which are designed for people that are heavier and uses higher density foam so these may be good value in some cases but again to know this you would need to know the quality specs of each layer which may be difficult to get.
This article includes my thoughts on the Heavenly mattress and all other mattresses from major manufacturers that use low quality polyfoam or other unknown lower quality materials in their mattress (especially in the comfort layers) and I would completely avoid them.
I’m glad you found us
Phoenix