Hi Dan1979,
If they were both the same type of latex then you would sink in more deeply on the 65 kg/m3 because it’s both thicker and softer.
I doubt that you would bottom out (reach the maximum compression of the material) on either one although it would also depend on your body type and sleeping positions. Thinner mattresses get firmer more quickly than thicker mattresses so it can sometimes feel like you are bottoming out even when you’re not because foam materials get firmer as you sink into them more.
It can be a good option if the mattress you buy is too firm and you need some extra softness and pressure relief. Of course it’s always better to be able to test the topper and mattress together because one affects the other when you sleep on them together and the same topper can feel different on different mattresses and choosing the ideal topper can sometimes be a little difficult but there are some topper guidelines in post #8 here that can be helpful if that’s not possible. The first step though would be the mattress so you can use your experience to decide if you need a topper.
I would think the 65 would still be a fair bit softer than the 80.
Phoenix