Hi Karim71,
Welcome to the Mattress Forum! 
I’m guessing you haven’t read yet the Mattress Shopping Tutorial on the site, as if you did you’d already have learned there is little point to going into Sleep Country because these are the types of stores that I generally suggest avoiding for many reasons including that they don’t sell good quality/value mattresses (see the guidelines here ).
Unfortunately, the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors, and these will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with most of the retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see post #3 here and post #12 here and post #404 here ).
However, if you can find out the thickness and density of all the layers and components in that mattress (which is unlikely to be provided to Sleep Country by Sealy) and post them on the forum I’d be happy let you know if there are any lower quality materials or weak links that would be a reason for concern. While I don’t have access to the completed specifications of this particular mattress, Sealy tends to use 1.5 lb density polyfoam for the base core in the Optimum line, which would be below the 1.8 lb minimum I recommend as a baseline, and certainly lower in density than what you would expect in this price range.
Regarding using online “reviews”, I would tend to avoid them as a reliable means of analyzing a product. While other people’s comments about the knowledge and service of a particular business can certainly be very helpful, I would always keep in mind that you are the only one that can feel what you feel on a mattress and I would be cautious about using anyone else’s suggestions, experiences or reviews on a specific mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you. In many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews or other people’s experiences in general won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress for any particular person (see post #13 here).
If you decide you want to look online, you can use the experience and expertise of the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices, and there are some who specialize in memory foam and have brick and mortar stores in Canada and ship in Canada. I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency. There is also a list of other online mattress sources for Canadians in post #21 here .
If you’re interested in shopping locally, you can let me know your town and I can see if I have a listing for some of the better retail options in your area.
Phoenix