First of all, if you went from 1.081 to 1.020, you're doing fine. That's 75% attenuation which is middle of the road.
As for the maple syrup, the label I have in my hand says 212 grams of sugar per cup, or 0.47 lbs or roughly 20 gravity points based on 44 points per pound for sugar (per gallon). So divide by 5, this adds about 4 points to a 5 gallon batch. So figure an effective OG of 1.085.
And yeah, next time make a starter.
Hope that helps,