I got about 420 km per tank taking it easy, 380 km (at worst) giving it a bit from time to time (maybe a bit more often).

Since the Sprint Booster, I regularly get 370-390 km per tank.

I tested the first tank with the Sprint Booster and got 280 km from that tank but I got fuel from a different station to usual- not sure what happened besides that- never happened since.

I think the Sprint Booster is a good, cheap mod. A friend interstate has a BMW M3 and he says they have a "Sprint Booster" sort of thing built in - press a button to turn it on and off.