Hi AcGolfer.
I am glad you asked before going down a rabbit hole as I am not sure that fixing a mattress that is starting to show issues is the way to go for several reasons…if the condition or design of the mattress itself is part of the problem (other than comfort layers that are too thin or firm) and has either significant impressions or soft spots under certain areas of the body or if the comfort layers of the mattress are already too thick and soft … then a topper will only follow the dips and soft spots or will be even thicker and softer and will provide only partial or temporary benefits at best and in some cases can lead to even worse problems with alignment. Like most of the major brands … the Serta iComfort has poor value and it is made to cater to the highly subjective and managed environment of a showroom floor but this has little to do how a mattress will feel and perform in the long term. There is more about many of the iComfort mattresses in post #11 here
I would generally suggest avoiding a fix with a topper and start your mattress research from ground zero with the tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choices … and know how and why to avoid the worst ones which would include the major brands such as Serta or any mattress where you aren’t able to find out the quality of the materials inside it.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well he will sleep), durability (how long he will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
When assessing any product, also be sure that you find out information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
Phoenix