Using maple syrup comes up often. In general, the maple flavor largely disappears despite how much you use.
I have included lots of maple syrup in the primary for flavor, and tried to prime with it. What others said was true, the maple flavor pretty much vanishes in both cases. The sugars are very fermentable so they do raise the final ABV, and have the same effect on flavor as adding straight table sugar - a thinner beer taste with higher alcohol.
Legally, maple syrup is defined as 66% sugars, the rest is water and a tiny bit of maple essence. Use 1.5X maple syrup for the same content of dry dextrose, sucrose, or maltose.
I personally have had more luck using maple flavor extract to give that unique flavor. Extracts vary widely. I used 1 tsp in a quart of unfermented wort. That was too much. Then I added fragments of that wort to another quart until the flavor was wher I wanted it. Then after fermentation the flavor was too intense and mellowed over a month.
Overall, I think maple flavor is best done with extract. The amounts for one brand of extract will not be appropriate for another. Using the right amount of extract at the right time is art and experimentation.
I save expensive maple syrup for my pancakes.