100% tires.
I highly doubt the brakes (calipers and rotors) are undersized for the car. The calipers and rotors are like heatsinks for the braking system. A well design system will be able to take multiple repeated stops without overheating. You could try to use a brake pad with more "bite" but it won't really slow the car faster because more than likely the pads will cause the tires to lose traction sooner resulting in ABS engagement. Once the ABS comes on, your stopping distance will lengthen. (Assuming you are using the same "not so sticky" tires)
Stopping distance however, it going to come down to the tires contact patch with the road surface and stickiness. Throw a stickier tire on the rim and you'll see your stopping distances plummet. This is the draw back when chasing range, you can't have you cake and eat it too. If you pump your tire up, it reduces contact patch, which is you get more range because rolling resistance is reduced. But when you ask the tire to do work, cornering/braking/accelerating, this reduction in contact patch results in a loss in performance. The same goes for using a less sticky tire or a tire with a higher wear rating, these tires don't "grip" as well, don't wear as fast. This can result in lower rolling resistance and better range, but again when you ask the tire to do work it will fall short when compared to a stickier tire.
As for 19 vs 21 vs any other size. It comes down to the tire used. Personally, I always go with the smaller wheel because tires of the same spec will always be cheaper. Very rarely will you see a 21inch version of tire A be cheaper than the same version of tire A but in a 19inch size. And of course wheels of the same design, using the same material will generally result in a 19 being lighter than a 21, which results in less unsprung weight. But I'm starting to get off topic now.
As for the Tesla, I think the Plaid is using PS4S tire, which is a VERY VERY good well rounded summer tire. Some might say it is one of the best if not THE BEST choice for an all around summer tire. I've never driven a car on the P-Zero HL tires, but I can tell you that the PS4S is not marketed as a high comfort, low rolling resistance tire. As a result comparing 60-0 distance between these two tires is not a fair comparison at all. I'm also not sure which model P-Zero comes on the 21inch wheels.
It would be interesting to see what the 60-0 stopping distances would be if the Lucid was rocking some PS4S tires though, maybe one of the snazzy youtube journalists reading these forums posts might give it a go. You know.....for science!
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