I might be an outlier, but all Teslas feel the same to me as far as stiffness and bumps go, when driving in a straight line. The difference is that in turns, an 85/85D feels more "wallowy" compared to a P85+. Try for yourself - drive 65mph on a highway, and do a hard lane change. The 85/85D, once in the destination lane, will squirm, or bounce once or twice, then settle. The P85+ will stay level, and planted.
If you drive normally, you won't really notice much of a difference. That is, to appreciate an M5, you must drive it hard enough so a 528i would struggle.
Then again, I'm the biggest P85+ nut there is, so take what you will.