As someone who has done
extensive modifications to a P85... I can assure you that there really isn't much, if any, software-only headroom there. Even if there was the full support of Tesla's software team to do the software-side modifications needed, any increase in power would, at a minimum, require upgrading the battery pack to support a higher current draw... essentially would need the Ludicrous retrofit. No battery packs produced in the P85 era are capable of increased current draw without some pretty in-depth hardware modifications. As an example, the procedure for the P85D to
P85D retrofit is something like 40 pages long and requires at least 4 hours of labor with two techs, plus prep time. Some older P85 batteries would require even more modification.
Additionally, if by some miracle a P85 still has an older drive unit that's never been warranty swapped (or replaced before Tesla revised the inverter), then the drive unit would need to be upgraded to handle the higher current.
To be able to handle the added torque on a launch without spinning the tires, the traction control system would need an upgrade to the current generation system as well (ESP2 + iBooster + new sensors, etc) which, by the book, would require replacing one of the main wiring harnesses in the car since, as a policy, Tesla will not modify them.
Overall, no... there is not really any software headroom left with the P85, and a performance upgrade package put out by Tesla is pretty unlikely. Overall, I'm pretty sure anyone with a RWD pre-AP car has gotten everything they'll ever get, really. People with AP1 cars are pretty much there also. With the exception of minor firmware tweaks here and there, if you own a "Classic" Model S, I wouldn't expect much of anything from Tesla anymore. Plus, every one of these cars will be out of warranty by this time next year at the latest, minus the battery and drive unit. (Extended warranties don't really count, it's basically just an expensive service contract/insurance with a decently high deductible).