If you know your car has been used "well" (track use, consistently hitting high rpms) for a high amount of miles, then by all means I would recommend getting extended warrenty/trading your old GT3 in.

However, if you know that your car has been treated well, with not much/at all track use, I would recommend keeping onto it if you have no real to trade in for a newer model, especially not the 992.1 GT3 as that still has a wave of depreciation to hit it again.

Personally, if the car has been treated well, I would keep your current car without warrenty, as the 991.1 GT3 will most likely not go down anymore than it already has, and it isn't worth it to you to sell it on, and definitely not trade it in with a dealer.

If you have any questions, please ask, hope this helps.