I dont know the statistics for boston, I do know generally big law is top 1/3. With some special circumstances (great soft factors, IP, etc) you can maybe go a little lower and still get in. I only know two people that applied for boston firms (one superstar that got crazy offers and one transfer student that got offers). I would say top 1/3 and you wont have many problems.
As far as making top 1/3, GW isnt that competitive. Everyone I have met, except maybe a few, are really chill. I would say about a third of the class doesnt care too much about grades, or does care but doesnt try anyway. So, the competition is only with about 2/3 of the class. I think a lot of doing well comes down to work ethic. I work my butt off and have consistently been in the top 20%. My friends are on top of things as well and all seemed to be in the same range.
If you come in and do the work, bar some really big distractions in your life, you really shouldnt have a problem being in the top 1/3. However, this means you have to outline and have to read for class and take notes.
Anyway, it is a really fun school and you should be alright in Boston. You are going to have a blast here. Law school seems really intimidating, but is actually pretty chill, at least at GW. I know most of what will be the 3L class and a lot of what will be the 2L class and they are all pretty chill (some exceptions).
I dont want to study for exams, if you want any practice, you are more then welcome to come to DC early and figure out crim pro for me