I think you guys are only looking at one half of the equation here. Are you going to lose your ass selling the asset? Unfortunately yes. That's just the way life goes with luxury cars. BUT, you're essentially buying the same asset again which means that same depreciation works in your favor on the buying side. In the case of selling one car to get the same car I'd ignore depreciation altogether and just look at out of pocket cost.
During my shopping I was offered a 2014 very well equipped 2014 P85 w/ Autopilot for $75,000. I was also offered an S85 w/ AP for $65,000. Obviously it takes a bit of work to find a deal like that but in terms of looking at total out of pocket cost, you're now talking a difference of $10,000 to get out of your car and into an AP car if you can be patient finding a car.
You've got one half of the equation down. You've found a good price for your P85. Get that sold, get the money in hand and then find your AP car.