I sympathize with you...
Goodness, we purchased a pillow-top ONE year ago and it sunk like nothing I've ever had the misfortune of sleeping on. Since it was under a year they replaced it this past weekend. They wanted us to go with another pillow-top, but I said no way, not in a million years.
You can try flipping it; I mean it's yours, but I have no idea what it would feel like. I was ready to sleep on the floor to avoid our one year old mattress that also begins with an S...
You might try checking the warranty again. I spent an hour on the phone and then, we had to go through a bunch of paperwork and take specific pictures with a ruler showing the sags. (They offered having a technician come out for $35 to do an inspection, but I didn't want to risk more money on a terribly bad mattress.)
Our bed looked like a cursive w, and no amount of rotating helped. We went with a cheap, simple firm mattress = hard-rock and topped it with a down-alternative mattress pad. We did have to pay for delivery, but I wasn't willing to spend another dollar on a mattress from that company.
We have one mattress company locally that still makes a flippable mattress; that will be our next try if this one doesn't work out.