Hi MDS0617,
The two main benefits would be the added thickness and as you mentioned the additional layering options you would have available to fine tune and customize the comfort and support of the mattress with the extra layer. There is more about the effect of greater thickness in post #14 here.
When you can’t test a mattress in person then the most reliable source of guidance is always a more detailed conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart (such as Flobeds) and who can help “talk you through” which of the the different options they have available would have the best chance of success based on the information you provide them, any local testing you have done, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs to different body types, sleeping positions, and preferences than anyone else.
Phoenix