Sounds like you have one year left on the lease, and expect to have about 54K miles on her, which would be 18K over. I assume you got about a 15¢ a mile overage charge...which comes out to $2700.
So, figure out what your buy-back amount is. Subtract from that $2700...and whatever that amount is IF you can get a different ST that is same year, same miles significantly cheaper than that, I would...especially if you can get one at that price that is new or with fewer miles.
Look around.
As for the warranty, given that you vehicle will have about 54k miles IF you plan to keep it much longer after that, which I assume is a given since you will be buying it back, then go for it.
However, I can't stress this enough, the economics of keeping a leased vehicle, even when paying close to $3K in mileage overage, rarely works in ones favor vs just rolling over into another lease, or turning in and buying new or used.
TJR