"I feel like at 55º, my clothing might even be enough... but I'm sure this bag will be warm enough for summer summits in the northeast ..."
I've been backpacking northeast summits for many years, and would never go out with a bag not rated at least to freezing. At the freezing point, you can rely on the insulating clothing to keep warm. If you're doing the LT, as suggested by another poster, it can get very wet also. It runs right through a snow belt, and summer rains can go for many days on end. The shelters on the LT won't add much warmth, but at least get you out of the rain with lots of space to cook etc. In my experience, the northern sections tended to be rainier than the southern, absolutely no idea why.
As to your question, I think the EMS Velocity, with PrimaLoft fill and a Pertex Qauntum shell, is as good a synthetic bag as you'll find without paying a premium price. I was hoping your thread might bring out something as good and lighter, though. Or perhaps, you may try the other posters' suggestions to use a quilt.
Best wishes for a great backpack, and please don't totally rule out The Cohos Trail.
It's not quite as rainy, is a lot less crowded, has some great lodges along the way (if you can afford lodging - around the same prices as motels, and often there are discounts for hikers), and some truly off the beaten path locations, like the Baldhead shelter, one of three north of the National Forest to the Canadian border.
Disclosure: I am affiliated with The Cohos Trail Association (volunteer work only).