Hi Kevin,
The support layers in a memory foam mattress are an important part of the performance and feel of a mattress and the thickness, quality, and firmness level that would best meet your needs depends on your height, weight distribution, your sleeping positions, and the thickness of the layers and the type of layers above it as well.
If you have an open and transparent manufacturer that truly has your specific interests in mind and that knows how to “fit” the mattresses they make to each person with a “reasonable” degree of accuracy, then they are the best people to answer this question. This is why I usually recommend either purchasing a mattress from a manufacturer who is what I call “mattress people” or from an outlet that offers a good refund or exchange policy to compensate for any mistakes you may make in your choices.
The base layer needs to be firm enough to keep you in alignment in all your sleeping positions and flexible and adaptable enough to adjust itself to all your different sleeping positions. People who are heavier tend to need thicker and firmer base layers to prevent the feeling of “bottoming out” or to prevent the mattress from feeling too firm. All of this also interacts with the type and thickness of the upper pressure relieving layers. A “typical” layering would be in the range of a 2-1 ratio (say 3" of memory foam over 6" of support foam) in terms of base foam thickness compared to the comfort layers as a starting point but there are so many variables in both people and mattress materials and construction that this is only a starting point for for a “typical” memory foam mattress that uses a single layer of memory foam over polyurethane support layers.
There are no exact “rules” because each person has many differences in their weight distribution, their sleeping positions, and in their preferences (how they want a mattress to “feel”) and the source of your information about any specific mattress, the knowledge and reliability of the people who are giving it to you, and your options if you make a mistake are more important than going by a specific standard that may apply to someone else but not to your own circumstances.
Thinner base layers will be firmer than the same foam in thicker layers and are also better able to deal with heavier weights. Thinner comfort layers will also allow you to feel more of the qualities of the foam or materials below them. The best thickness and softness of the comfort layers will also depend on your height/weight and sleeping positions and your own testing will always be more accurate than “theories” which can be a good starting point but not an “ending point” or a great basis for a final choice that is most suitable for you. In essence … you will need to trust or not trust the knowledge and openness of the person who is advising you and the source of the information about how suitable the specific mattress you are considering will be for you. Without this … return policies are one of the most important factors because the odds of making a mistake are considerably higher. You can see from reviews that what is “one way” in terms of firmness, support, and feel for one person is very different for another.
So my goal is to help point people in the direction of what factors to consider and “how” to make better choices rather than to make specific choices for them. I personally believe that local manufacturing and/or better educated sleep shops that carry alternative brands are usually the best choices where the odds of making good quality and value choices are much higher than anywhere else.
If you do want to make a purchase online though … then asking good questions (about materials, their recommendations about the mattresses you are looking at, and the policies of the outlet if you make a mistake in your choice) and using your best judgement about the accuracy of the answers will also help tilt the odds of making better choices in your favor.
So you are looking for one of the hundreds of “gel memory foam mattresses” where the person you are taking to when you are looking for information is clearly knowledgeable about the materials in their mattress and also about how to fit a mattress to your needs and preferences. As you say I would tend towards the gel infusions rather than the particle infusions or towards versions where there are “cylinders” of gel foam placed inside the memory foam rather than particles. I would also ask good questions about the support layers of the mattress and whether the person selling it believes it would be suitable for you.
Without the help and knowledge of someone who knows the specifics of the mattresses they sell and the ability to help you make your best choices based on their knowledge of how their mattresses are made and the materials it contains … you are at the mercy of their refund policy which would be my most important consideration unless I knew for sure (through research or specific local mattress testing) that the mattress I was buying was exactly what I needed and wanted and that the value was appropriate to what I was paying.
One final comment is that if your preference for “gel memory foam” is based on your testing with the iComfort … it is worth noting that unless your preference was for the Insight or the Goodnight refined models (which are the only ones where you are actually lying on gel memory foam) that the feel of all the other iComfort models have less to do with the gel memory foam than they do with the materials above and in between the gel memory foam than with the gel memory foam itself which is only part of the “mix”. Every one of their mattresses except the insight and goodnight refined has at least 2" (and mostly more) of either “regular” memory foam or other foams in the comfort layers either above the gel memory foam layers or immediately below it (besides the support foam) which are as much or even more responsible for how they feel as the gel memory foam itself. It’s also worth noting that as with all materials … the different versions of gel memory foams do not have the same specs, feel, or properties and that the layers they are combined with also have a significant influence on the performance and feel of your mattress.
Phoenix