Hi tempured mad,
This is one of those questions where the only meaningful answer would be “it depends”. If adding a topper results in an improvement in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then it certainly would be but if it doesn’t then it wouldn’t and unfortunately the only way to know for certain would be to actually sleep on it.
As you may know from your reading it’s much more difficult to firm up a mattress that is too soft than to soften a mattress that is too firm because even a firm topper will still sink down into the softer layers underneath it and in most cases the most reliable way to make a mattress firmer is to replace the layers that are too soft with thinner or firmer materials. In some cases adding a topper can make an alignment issue worse.
With memory foam though if you add a topper then it would reduce the amount of heat that reaches the memory foam which can either reduce or slow down the softening of the memory foam with body heat to some degree so it may have a partial or temporary effect on firming up the memory foam. Thicker toppers will have a bigger effect on reducing the amount of heat that reaches the memory foam and on reducing or slowing down the softening of the memory foam than thinner toppers. With thinner toppers you will “feel” more of a combination of the properties of the topper combined with the memory foam underneath it so with a latex topper you would have a more resilient sleeping surface with some of the slow response and sinking in from the memory foam underneath it while with thicker toppers you would have more of the feel of the topper material itself that was modified less by the memory foam layers underneath it.
If you do decide to try a topper then it would be well worthwhile to factor in the exchange or return policy of the retailer or manufacturer you are dealing with just in case it doesn’t work out as well as you hoped for unless you have a use for a topper regardless of whether it turns out to be an effective solution for your mattress.
There are also some suggestions in post #4 here (including the link to posts #5 - #7 here) that may be worth trying with a memory foam mattress that is too soft before you go to the expense of adding a topper.
Phoenix