My understanding is that American Express Extended Warranty simply adds time to the original warranty, but doesn't change the terms. If the original warranty is still in force, American Express is not involved. If the original warranty has recently expired American Express "should" apply the same standards as the original warranty.

However, American Express will extend the warranty only if the original runs five years or less, and I suspect the original warranty may be longer.

I think this is an instance where careful selection of the product is important. For example, a mattress style which can be "flipped" may spread the wear more evenly. I would also consider buying from a merchant with a reputation for liberal returns or exchanges, such as Costco or Nordstrom, but I have no direct experience.