I put a quart of grade B syrup in 5 gallons of porter and liked it. Post-fermentation, the syrup has more of a woody than typical maple flavor. To maximize maple flavor, I'd recommend any combination of:
- add syrup to fermenter after peak krausen
- ferment as cool as possible
- residual sweetness should bring out the maple, so reduce IBU
- go easy on flavor/aroma hops, they'll compete with the maple