The P85D with + suspension has different sway bars, staggered rear tires and various other suspension parts that improve the handling dramatically... this was the big draw to a P85+ (which is where this suspension was introduced).... a standard P85 IMHO seems less "planted" on the road and tends to porpoise upon significant acceleration. some say that a NON + car is a slightly smoother ride but I think the air suspension balances that out..When the P85D was first introduced (in the first 6 months) most were built with the same suspension parts that the P85+ had. In my opinion the P85D+ is the best car made because it has the advantages of the D, it is significantly faster, it has AP1,, next gen seats AND it is a better handling car than a later version of the P85D and any other model since. My P85+ handles as well (if not better) than my 911. (2 totally different cars) In short it is the best of both worlds.
As someone mentioned there was a time that you could upgrade to a + suspension for about 13K but they stopped doing that long ago.. a few months back Tesla changed the 21" rim option and now all 21" rims (on new cars) have staggered tires. But still they do not have the other + components. Again my opinion, but a car with this much power should feel "planted" or "sure footed" under hard acceleration and so forth.
Without getting too long winded that is the reason why I (and others) say the P85D+ is the best Tesla made. It is the best combination of speed and handling.
The degree to which you can truly appreciate this difference depends on your driving habits... and how far you push the car on a regular basis. If you are more of a cruiser then the + suspension is not as much of a wow but for those that are more "spirited" drivers, the car feels more predictable, and maybe even safer... Others have used the term "on rails".
To clarify the ESA thing... it used to be that only the original owner could buy the extended warranty.. Now as long as the car has never passed through a dealership any current owner could buy the extended warranty. So the determining factor is that it must be a private sale vehicle that has never been through a dealership... when I bought my P85+ used 2 years ago I bought the ESA and started out with 6 total years of warranty and up to 100K in mileage... In some cases (like mine) this works out better than buying a certified pre-owned car from Tesla where I would have gotten 4 years and 50K miles tacked on to the current car mileage... Also some states like Nevada do not charge sales tax on private sale used vehicles so that saved me about 6K on top of everything else...
Lastly, there are a lot of threads here on the merits (and drawbacks) of the + suspension. they might be helpful to read as well.
Hope that helps