Hi nucappy.
Welcome to our Mattress Forum. 
It sounds as if the alignment of this mattress (with the new topper) agrees with you more than others you tested, so that’s a good thing. While I can’t diagnose why your arms are feeling numb, I can provide some insight and possible things to investigate.
Technically, limbs going numb are usually caused by a lack of pressure point relief.
the upper layers are about “allowing” enough sinking in to create the mattress cradle which is a larger area of surface contact with the mattress to relieve pressure and support the recessed areas of the spine. The deeper layers are about controlling any further compression than necessary for pressure relief to “hold up” the heavier parts of the body and keep the spine in its range of natural alignment. The amount of sinking allowed by lower layers has less effect on pressure relief in other words (in most types of mattress construction) but controls alignment. This is why when people change the firmness of upper layers to solve support issues or the lower layers to solve pressure relief issues they will often create new issues and may not solve the issue they are trying to “fix”.
Arms falling asleep often indicate a comfort layer that needs to be softer or thicker. The next most common reason is probably a pillow issue (likely needing a thicker or firmer pillow) which could also affect pressure on the shoulders. Of course the initial break in or adjustment period is also a possibility but this would generally lead to gradual improvement over time … particularly if you walked on the mattress to speed it up. The challenge of course is to figure out which it is so that any fine tuning or refinements can change things in the right direction. Sometimes people are sleeping better throughout the night and repositioning less often on their new mattress, and this is the cause of pressure build up.
The links contained in post #11 here provide quite a bit of good information about diagnosing possible reasons for should joint pain.
There is also more information in post #2 here that talks about the various symptoms that people may experience on a mattress and some of the reasons that can cause them that can help with the detective work or trial and error that may be necessary to identify the types of changes that may be helpful.
This is indeed a risk. You may get the feel you are looking for…for a while, until it starts to sag. If you can get the specifications of the S brand you’re looking at and compare it to our Durability Guidelines that can help you to narrow your options.
Interesting What kind of bed frame do you use? Do you know what kind of frame it happened to be on in the showroom?
Have you considered a latex hybrid? Latex is great at offering both comfort and support, and sleeps cooler than the other foams.
NikkiTMU