I'm pretty sure the engine didn't come on (nor did the display drop from 4 to 3 chevrons like at high speed). There was still a significant regen braking feel, and I didn't have to ride the brake pedal the whole way down, just occasionally to slow for each curve and on a few very steep straight sections.
I recall a review of the Bugatti Veyron saying it could suck down about 1 gallon per minute at high speed of 220 plus, so a tank is gone in a little over 20 minutes.
How insulting to believe the Bugatti Veyron is so fuel efficient! Automobile Magazine says the regular old Veyron's 26.4-gallon tank lasts only 12 minutes at the car's 255-mph top speed. The more powerful Grand-Sport Vitesse can suck the tank dry in a mere 10.3 minutes. Automobile adds, "Michelin won’t guarantee the tires to run above 250 mph for more than 15 minutes." A new set of tires (which must be changed as a set) will cost $30,000 and after the third tire change, a new set of wheels is required to the tune of $120,000.