Hi Smytrous,
I don’t know what could be causing the heat sensation and most people wouldn’t have any temperature issues with this mattress with the wool quilted cotton cover (which is typically very temperature regulating in both directions) but it wouldn’t be a normal “symptom” of a latex allergy no. It could be from your mattress protector or the sheets and bedding you are using. There is more about tracking down temperature regulation issues in post #2 here.
As far as I know the Eco has 2" of N2 Talalay (between 20 and 24 ILD) on top of a support core that is about 36 ILD and the Sustain has two 2" layers of N1 (between 14 - 19 ILD) on top of the same core so for most people adding a 3" N1 topper would be “somewhat similar” although it wouldn’t be exactly the same and since these types of comparisons are fairly subjective and can depend on body type, sleeping positions, and individual perceptions, different people may have different opinions about how closely they “match”. The most reliable way to know how they compare or which one would be the best match for you in terms of PPP would be to compare the Eco with the topper and the Sustain in the store so you could use your own perceptions and experience to compare them.
There is also more information in post #2 here which may be helpful when you have a new mattress that is too firm.
The advantage of a separate topper … especially if it is very soft … is that you can replace it without having to replace the entire mattress because the top layers of a sleeping system will tend to soften and break down faster than the firmer support layers and components.
Phoenix