The more interesting use case for supercaps is in the 60-0 case or in the case of delivery trucks, 30 or 15 to 0. The inefficiency is in capturing the braking energy. If you can use supercaps to capture this inrush then you can trickle it back in to the battery pack. You don't need the capacity to capture all the braking energy nor do you need supercaps to replace the battery. You just need enough buffer to improve your overall efficiency. In something like a delivery truck the battery/cap ratio could even be adjusted with modular packs depending on route. I'm imagining something similar to what Gogoro scooters use but with two kinds of pack.

0.5 * 1850 kg * ((60 mph)^2) = 665 483.056 joules

Supercaps are 28800 Joules per liter, so you'd be needing nearly 700 liters of them to store the energy of the car decelerating from 60 mph once.

700 liters is a lot of volume in a car where you already use a lot of the available volume for batteries - it's more than the trunk and frunk combined on a Model3.

That might still be doable for a delivrry van or a bus, even if it is heavier. Natural gas powered already have quite a big "hump" on top for the pressure botzles with gas, I could imagine putting the supercaps there and the regular batteries inside the chasis to keep the center of gravity low.

0-60 certainly is not the important metric for this application, perhaps aside from rural routes. Most residential delivery, you're maxing out at 30mph.

Correct, and given that the velocity is squared in the energy equation, this is an enormous difference.