6-7% gain, estimated
So, I drove with the alternator off [simply disconnected] for probably 90% of the last tank.
Tank MPG was 9% higher than the previous tank, with similar trips.
That said, the weather was a fair bit warmer, so we can't attribute all of the 9% gain to the alternator situation. Ambient temperature has a big effect on fuel economy.
Let's be conservative and call it 6-7%. Seems reasonable.
One thing I noticed is my driving style [mostly country secondary highways, next to no traffic] would benefit very little from having the regen feature.
View my fuel log 2014 Mirage SE 1.2 manual: 63.0 mpg (US) ... 26.8 km/L ... 3.7 L/100 km ... 75.6 mpg (Imp)