Someone worked it out in 2007. The "turf advantage" for a "turf team" works out to 0.2 goals per game, or 1 goal every 5 games. If you play 15 games on turf, that works out to 3 extra goals. Enough of an advantage when standings are tight.
The ideal situation would have the lads practice say 3x per week on grass and 1x per week on turf. That way you stay familiar with the turf, but don't get the wear and tear from constant practice.