Ok, so I was bored and wrote a quick finite element analysis of coasting vs regen just to see what happens. The scenario:
Coast to a stop and calculate distance traveled
Calculate the energy used if you keep going at constant speed and use max regen to travel the same distance
Work out how much more (or less) you use with each method.
Parameters:
weight = 1450 (car plus driver)
motor efficiency at constant speed = 95% (from i3 efficiency graph)
regen efficiency to battery = 75% (internet)
accessories = 2kW (air con/radio/car systems etc - from personal data)
air density/Cd/A from usual sources = 1.2kg/m^3, 0.3, 2.1
tyre rolling resistance = 0.01
dead flat road, no wind
Speed 150kph:
Coast for 3.305km, taking 214s to come to a stop.
Constant speed, max regen (0.19g) to travel 3.305km takes 90s
Difference in energy used: 0.35kWh more than coasting
Speed 100kph:
Coast for 2.135km, taking 180s to come to a stop.
Constant speed, max regen to travel 2.135km takes 84s
Difference in energy used: 0.105kWh more than coasting.
Speed 60kph:
Coast for 1.056km, taking 130s to come to a stop.
Constant speed, max regen for 1.056km takes 67s.
Difference in energy used: 0.02kWh more than coasting
At 30kph it was 0.005kWh, so basically the same.
So what does this mean? The faster you go, the more you lose to friction and the better coasting is. The slower you go, the less difference there is. At around 30kph there is effectively no difference. The next thing is how often do you get to coast for 1/2/3 km? If you can plan ahead for road works etc, and you don't mind annoying people behind you towards the end of the coast, then you will save a small, but measurable amount of energy. The slower you go, the less difference it makes.
I will note that if you are stopped at a traffic light, then the regen method will be sitting there waiting for the coaster to get there, and using accessory power, so it will get worse.