Hi Nemix,
Every layer and component of a sleeping system (such as a mattress/topper combination) will have an effect on every other layer and component above and below it yes. The only way to know for certain whether it would make “enough” of a difference or whether the combination will be a suitable “match” for you in terms of PPP (Posture and alignment, Pressure relief, and your own Personal preferences) though will be based on your own careful testing or personal experience on the mattress/topper combination.
If your mattress is too firm then a topper can certainly be an effective way to add some additional softness and pressure relief although there is always some risk and uncertainty involved in adding a topper if you haven’t tested the combination in person because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
Although this can be a good strategy if you can test the mattress/topper combination in person before a purchase … you can see my comments about choosing a mattress with the intention of adding a topper in post #2 here. In general I would avoid this approach because of the uncertainty involved with two purchase choices instead of only one and choosing a topper that would be suitable in terms of PPP for a specific person on a specific mattress can sometimes be almost as difficult as choosing a mattress that doesn’t need a topper in the first place. I would generally focus on choosing a mattress that is likely to be a suitable match without a topper (again unless you can test the combination in person) and then use the option to add a topper as a “backup” strategy in case your choice doesn’t turn out as well as you hoped for rather than a “primary” strategy.
If a mattress/topper combination is a good “match” for you in terms of PPP then it does have the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (the upper layers or a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time and a topper can also help extend the useful life of the mattress underneath it as well.
If you do purchase a mattress that ends up needing a topper then there is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable supplier that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of and a link to the online suppliers that have good exchange/return policies as well.
Some of the better suppliers I’m aware of that ship mattresses and/or toppers across Canada are also listed in post #21 here.
Phoenix