Hi HoosierLife,
A 2 hour trip may be well worth it since you will be sleeping on your mattress for many years.
It would depend on the specifics of the topper and the polyfoam and the mattress you were comparing it to but the upper layers of a sleeping system will affect how it “feels” more than the deeper layers so if the topper is the same type and blend of latex and the same firmness level as the top 3" of a mattress you are considering and the polyfoam is close to the same firmness as the deeper layers in the mattress you are considering then it would give you a reasonable sense of the pressure relief and “feel” of the mattress you were comparing it to.
I would also keep in mind though that every layer and component of a mattress (including the specifics of the cover) will affect the feel and performance of all the other layers and components and the mattress “as a whole” and adding a topper on a mattress will also be different from a mattress that has the same layer inside the mattress cover itself (see posts #3 and #4 here and this topic) so having a “general sense” doesn’t mean that they will be the same and some people will be more sensitive to the differences between two different “sleeping systems” than others.
Phoenix