Hi ashbosity,
The first place I would start is post #1 here which has all the basic information, steps, and guidelines that you will need to make the best possible decision.
Post #3 here has more about matching one mattress design to another. In the case of any of the iComforts the only way to match it would be in terms of “feel” and would require side by side testing and comparisons with another mattress based on subjective perceptions which can vary widely from person to person. I don’t know of another manufacturer who makes a mattress that uses the iComfort as a “target” for their design.
I would tend to avoid buying a mattress with the intent of adding a topper unless you can test the two together. A mattress can be a difficult enough choice and buying a mattress only to then have to go out and buy a topper which can also be a difficult choice would only introduce more variables than are necessary. Adding a topper can be a good backup if you make a mattress choice that is too firm but I wouldn’t buy a mattress with the intent of adding a topper and would focus on just a mattress by itself that was as close to your needs and preferences as possible.
There are many memory foam mattresses that have the same or better quality as the iComforts that sell for considerably less and would have similar levels of comfort/pressure relief and support/alignment even though they may “feel” different because of differences in design or the specific properties of the memory foam (there are hundreds of versions of memory foam that all have some similar slow response properties but are also quite different in many respects as well (you can read more about some of the differences between different types of memory foam in post #9 here).
Phoenix