Hi jumpingjones,
Welcome to the Mattress Forum! all the way from Australia.
I’m glad the information here was helpful … and congratulations on your new mattress 
I’ve also added Town and Country mattress to the Australia list of manufacturers and retailers in post #7 here … thanks for the heads up!
There is also more about flexible slat systems in post #2 here and the posts it links to. Since a flexible slat system is an “active” part of a sleeping system (just like any other layer or component either in, under, or over a mattress that flexes or compresses when you sleep on it) the only reliable way to know whether a flexible slat system would be an improvement for you in terms of comfort, firmness, and PPP compared to a rigid non flexing support system under the same mattress would be based on your own actual experience when you test it or sleep on it in person and compare both types of support systems under the same mattress. Depending on the design of the mattress, for some people it could be neutral, for some people it could be an improvement, and for some it could be detrimental. The flexible slats would generally be less noticeable under a thicker mattress than under a thinner one or for lighter people than heavier people.
Having said that … it looks like the flexible slat system you tested the mattress on in the store has a softer adjustable shoulder section that provides some zoning in the “sleeping system” and if this is the case then your mattress is probably thin enough that the softer support section would help to “allow” your shoulders to sink in a little more which can help with alignment and to reduce pressure on your shoulders. Again though … the only way to know “how much” difference it would make would be based on your own personal experience but based on your feedback it sounds like the difference is enough that you can notice the difference in the store vs the same mattress.
There is also more about zoning in this article and in post #11 here and the additional posts it links to.
Since you seem to have noticed a difference it may be worthwhile returning to the store and doing some careful testing with the same mattress on the flexible slat system once again so you can use your own experience to help you decide whether purchasing it will make “enough” of a difference to justify the additional cost.
If you add a layer of polyfoam under the mattress it wouldn’t be zoned and would probably make less of a difference than the zoned flexible slat system and if you are looking for a less costly alternative it would probably be more effective to add a softer topper on top of your mattress than to add a polyfoam layer underneath it where any additional softness it provides would be much less noticeable than it would with a topper.
If the only issue with your mattress is that it is too firm then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for both of you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the combination.
Because a topper probably wouldn’t be zoned with a softer shoulder section it would be important to use a topper that was “just enough” in terms of thickness and softness to provide the additional pressure relief that you need but no thicker or softer because the risk of choosing a topper that is too thick and/or soft is that it could end up being too soft and lead to sleeping out of alignment and you could end up replacing one set of symptoms (pressure points) with another (lower back issues). If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
There is more information about choosing a topper post #2 here and the topper guidelines it links to (although the sources it links to are only for North America) which along with a conversation with a reliable and knowledgeable supplier that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market can help you use your sleeping experience as a reference point and guideline to help you decide on the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
A good exchange/return policy (although exchange or return policies with toppers are less common) can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
Phoenix