Keep in mind that anti-roll bars, while they help resist roll also couple together the response of the left and right sides of the car more strongly. If all roads were always perfectly flat we could run incredibly stiff anti-roll bars and never have any penalty (other than weight) and cars would corner almost perfectly flat all the time.
In the real world, anything that upsets a wheel on one side of the car will be reflected by an overly stiff anti-roll bar at the wheels on the other side of the car and the result can be less overall traction if the bar is too stiff. So there are practical limits on the stiffness of anti-roll bars for real world roads. The sizes of bars chosen by Tesla engineers and development drivers, including lots of time on real world roads, race tracks and in simulations, are very good and work quite well in the real world.
Also, since the center of gravity on Teslas is so low, well below the roll axis and axles, the car has unusually little roll to begin with.
Lowering springs lower the already very low center of gravity even further, which should make stiffer anti-roll bars even less incrementally beneficial.
Remember that roll is generated as a lever arm of the center of gravity (really mass centroid axis) through the roll axis of a vehicle. With both so low on Teslas, there is not much roll generated to begin with. Lowered with shorter springs (or better, lower spring perches as on coilovers), there's even less roll to be had.
Bottom line, stiffer anti-roll bars are probably not needed and very likely at a point of diminishing returns on real world roads. Which is probably why Sasha hasn't found them needed on a race track. And on bumpy, potholed streets (i.e. real roads), they could make ride and handling worse. The stock anti-roll bar sizes are very good, and their effect is greater on a lowered car than a standard height car. With an exceptionally low cg, a lowered Tesla would benefit from stiffer bars even less than other cars in general.
My favorite reference is probably the classic book Tune to Win by Carroll Smith. I highly recommend it for some fundamental knowledge about tuning race cars. At this point, Tune to Win has trained multiple generations of racers and engineers, very likely including many who work at Tesla and most of the other major car companies, particularly through Formula SAE and similar student competitions, etc.