Hi brittany741,
The Tobago doesn’t list the specifics of the material in the comfort layers (and they would almost certainly be low quality/density which means that they would be unlikely to hold their comfort and support in the longer term) but it is so different from the mattress/topper sleeping system that you ended up with that the only way to know how well they will “match” each other in terms of PPP would be based on your actual perceptions and sleeping experience. They are much too different to compare based on specs.
In terms of durability … a mattress will tend to soften, change, or break down from the top layers down (the top layers are more subject to wear and tear from compression forces) so at least you know that the top 3" of your mattress is a high quality and durable material.
The mattress you purchased uses 3.5" of “questionable” and most likely low quality materials in the upper layers which would certainly be a weak link in a mattress in terms of durability but your 3" topper will slow down the rate that these layers soften and break down to some degree so they would probably last a “little” longer than if you were sleeping on them directly. If you are looking at the uncertainty of buying a mattress/topper combination (see post #2 here) I would also tend to choose a “base” mattress that was much firmer than the one you chose because adding softer toppers on top of layers that are already fairly thick/soft can add to the risk of alignment issues as a result of comfort layers and toppers that together may be too thick and soft for you. Here again though … your own personal experience will always “trump” theory.
The coils are fine so they would be a good base but the only way to assess how a mattress feels or how close it would be would be based on your own experience because there are far to many unknowns, differences, and variables between them to even guess at how close the combinations you are building would be to the Hampton & Rhodes Select Tobago Pillow Top Mattress.
I hope your choices work out well for you and I’m certainly interested in your feedback once you have put your sleeping system together and have had a chance to sleep on it.
Phoenix