Hi qazy,
A latex support core is more durable, more resilient, more elastic, more adaptable to different weights and shapes and sleeping positions, more supportive (it has a higher compression modulus so it gets firmer faster with compression), more “natural”, and has a different more “springy” and responsive feel than polyfoam. It is a higher performance material. Of course it is also more expensive than a polyfoam core and for some people … a latex hybrid which has the benefits and “feel” of latex in the upper layers (the top 3" - 6" which are the most subject to wear and tear and contribute more to the overall “feel” of a mattress) is worth the cost tradeoff. For others it isn’t.
I doubt the polyfoam core of the mattress you were considering was a 3 or 4 lb base (I know you said it was difficult to remember). A higher quality would generally be 2-2.5 lb.
The idea of configuring an all-latex mattress as you’re describing certainly would create a product that uses high quality and durable materials, and would also be configurable for future adaptations, as long as you’d be able to achieve a comfort that you enjoy.
“Any good” would be subjective, and it would intend upon what you want the product to do. If you want something that is actually waterproof, then a water resistant item wouldn’t meet your needs. There is more information about mattress protectors in post #89 here that you may find useful, as well as this post.
Phoenix