Csj,
It sleeps very cool and summer has been very hot as well. It’s not super springy like a box spring, with my only other experience being memory foam the best way I can describe it is for our setup you sink into it similarly to memory foam except it feels way more supportive and it’s a bit more firm. The first few nights felt too hard for us but after breaking it in a bit it became much more comfortable.
No mattress topper, we just sleep directly on it.
The 13 inches choice was based on Sleep EZ’s pricing progression, for a king-sized bed it’s $2195 at 10 inches and $2495 at 13 inches, which makes the fourth 3" layer essentially half price. I figured if I’m already willing to spend 2200, then 300 more for 25% more mattress is worth it for the extra support, flexibility and durability for a 10+ year purchase.
One post that I found helpful was #14 from Phoenix here: https://forum.mattressunderground.com/t/memory-foam-toppers-do-any-good-ones-exist
That being said, it seems like 10 inches also seems like a great choice especially if you’re doing a size like twin where you don’t really get the extra layer at as much of a discounted value compared to the first 3 layers.