1.) Agreed, Northern New England can hold way more than 8ft, it gets huge up there. So can the Outer Banks.
2.) I don't consider Florida the Southeast, Florida is much more consistent than the southeast, swell window/exposure from both directions +exposure to tropical activity would explain that. However it still lacks the max size and power of the northeast.
3.) The continental shelf (south of the outer banks) has 2 levels, the primary one right off shore (shallow one), and the secondary one out further to sea which is deeper. From what I've read, it's the top level one that has the main impact on decreasing wave power, and the lower one is so deep the effects are very minimal. I can also sort of tell from living in several places on the east coast. East Florida gets pretty good waves and doesn't seem to be effected by it too much. (You can tell by looking at Reef Road Florida and the size it can produce) The shelf narrows as you go south in FL.
4.) Really just talking about power and size of the waves, consistency and swell exposure aside.