Hi JimA,
A king size is 76" x 80" which would be 4.69 yards and multiplied by 2 lbs/yd would be 9.38 lbs in a king size (although the 2 lbs is probably an approximation).
Your best source of guidance about a layer exchange would be a more detailed conversation with Brooklyn Bedding who will be able to use their knowledge and experience about their mattresses and other customers that are similar to you to help you decide on the firmness of a layer exchange that would work better for you and/or your wife. The more specific and detailed information you can provide them on a phone call about you, your preferences, your current sleeping experience on the mattress, and what you would like to change … the better they will be able to help you.
The cost of a layer exchange would end up being $75 for shipping which is much less than a topper.
If you do decide to add a topper … then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point to help you choose a topper that has the best chance of success.
Phoenix