If the biggest difference is the sway bars, does anyone know if Tesla will retrofit to a P85?
Tesla likely won't, and the difference may not only be the sway bars. The P85 shared air units with the other MS's (front 6006351-00-C, vs. P85+ - 6006351-04-A). If a P85 is early, like pre-7,000 VIN, it may also have the older sub-frame (a $3,700 Tesla job, if you search). The part numbers for early P85 and '+' don't give diameter for the sways, like they do the dual motor cars. They are, however, separate for the two RWD cars. I can confirm early/late dual-motor PD were 26/21 and later 24/21 mm.
I've had two P85+ loaners while my pre April P85D with + suspension was in for service. The P85+ is a more compliant comfortable ride yet it's handling is far superior to that of my P85D+. I own both 19s and 21s and neither make that big of a difference so it's not the wheels. The P85D+ is harsh and unforgiving yet it doesn't handle corners as well as the P85+. The only advantage so far is powering out of corners which is better because of the AWD.
Between your feedback, a couple used parts listings for the early PD air units and Tesla eventually dropping them, I don't think I'd want an early P85D SAS car. This is coming from someone who swapped out the lower control arms of a GT3, because they were too squishy. I don't think the firmness of the original P85D suits most, not in the strut anyway. Some like it, but having had a late loaner P85D, while my coil P85D was in for service, I don't think either is lacking in the transients. It's also not an affront, to me, that all dual motor cars share the same suspension (P100D manual updates notwithstanding). The roll center is so low that there isn't much travel to begin with. So, I am not sure the point in overly firming it up?
As a P85+ owner I could not agree more. I drove a loaner P85 and it felt almost "unsafe" or at least not "sure -footed"
I found the early P85's used to porpois pretty bad on-throttle, in turns. In contrast, the dual-motor cars don't squat as much, and basically pull themselves flat. So, low roll and AWD are two reasons not to mess with what I think has evolved into a good thing.
I'll still be curious what somebody says, if they ever swap the sway bars.