You may however be overestimating how much a car like the S needs to slow down in a turn. It has a really low center of gravity. Maybe it didn't need to slow down at all and you were just being paranoid.
Can you provide a map link to the stretch of road where this happened?
I don't think I was being paranoid....I'm generally a very, very fast driver who prefers the inner lane and 85mph+ speeds given the opportunity. I do know the car has amazing handling but it unequivocally started swerving heavily out of the lane at the speed it was traveling at.
It's route 90 between Allston and Back Bay. It looks like a pretty gentle curve so looking at the map certainly makes my description of events seem questionable, but I can only tell you that it occurred as I described.
But it is not 100% false that the Tesla I test drove functioned this way. It is, in fact, 100% true. This happened last month.