Hi Trailblazr80,
Welcome … and I’m glad you found us :).
A topper can be a good solution for a mattress that is too firm and that just needs some additional softness and pressure relief as long as it doesn’t have any soft spots or sagging in the sleeping surface but it’s not a particularly good solution for a mattress that is too soft or has developed soft spots or is sagging because a topper will just “follow” the soft spots or sagging underneath it. At best it may be a partial or temporary solution and at worst it can make any issues that you are having with your mattress worse. For example you could end up with a sleeping system where the top layer (the topper) was too firm for you to provide good pressure relief but the layers underneath your topper are either too soft or are sagging too much to provide suitable support to keep your spine in good alignment.
There is more information about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
The most effective way to “fix” a mattress that is too soft or that has developed soft spots or is sagging is to remove and replace the foam layers that are breaking down and not to add something additional on top of them. There is also more information and suggestions about mattresses that are too soft or that have visible or virtual impressions in post #4 here that may be helpful.
While it’s not possible to quantify the sleeping temperature of a topper or sleeping system for any particular person with any real accuracy because there are so many variables involved (including the type of mattress protector and the sheets and bedding that you use which in many cases can have just as significant an effect on temperature as the type of foam in a mattress) and there is no standardized testing for temperature regulation with different combinations of materials … there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
If you tend to sleep hot though I would tend to avoid memory foam which tends to sleep warmer than other types of foam materials.
If you decide to try a topper anyway then post #2 here and the topper guidelines it links to has more information and some suggestions that may be helpful (although they are designed to help choose a topper that can provide some additional softness and pressure relief and not to “firm up” a mattress that is too soft) and includes a link to the better online sources for toppers I’m aware of as well. I would also make sure that any topper you decide on has a good return/refund policy just in case it doesn’t work out as well as you hoped for.
Phoenix