Math says:
http://faculty.nps.edu/sebuttre/docs...oringRates.pdf
In the 2008-09 season, 5 on 3 resulted in 19 goals per every 60 minutes. This is 8x the scoring rate at 5 on 5. And 3x the scoring rate at 5 on 4.
That is a big enough advantage not to assume the risk of an empty net goal at 6 on 3.
On a two minute 5 on 3, you should expect to score about 63 % of the time.
A 56 second 5 on 3 should result in a goal 30% of the time.