Hi Petex,
I would first confirm that the bed you have would meet the manufacturer’s warranty criteria … especially regarding center support. I would also confirm that the Laxeby meets the warranty criteria as well because some manufacturers (such as Tempurpedic) have very specific criteria about the type of bedframe or foundation that is necessary and it may not include a base that has any flex.
If you have a flexible foundation under a foam mattress then even if it provides suitable support in terms of the number of slats and the gaps between them (which it would), it can also change the feel and response of the mattress. This can be beneficial or detrimental depending on how a specific combination feels and performs for you. As you mentioned you may also be able to feel the firmer center support through your mattress depending on the thickness and type of materials in your mattress, your weight and sensitivity, and on how much you sleep on the center of your mattress. These are all things that you can only know with your own personal experience if you are using a different type of support system than when you tested the mattress.
You’ve probably seen this but there is more about the different types of mattress protectors in post #89 here. Waterproof vs water resistant, the effect on the “feel” of your mattress, and how it affects sleeping temperature are all tradeoffs and there isn’t one that has the best of all of them. Most people are fine with the thinner membrane type but there are also some people who sleep warmer on them regardless of the specific brand (possibly in combination with some of the other factors that can affect sleeping temperature that are discussed here).
Phoenix