WarcraftRealms.com

I was wondering if there was a way to find a realm with the following qualities:

PvP
Medium Population
Equal Alliance to Horde ratio
Boston Datacenter
US Eastern Time
Horde dominated Battlegroup

Does a realm like this exist? Is there a way I could use this site to find a realm close to this? If not, does anybody know of a realm like this?

Also, would there be any disadvantages to being in a realm like this? If so, what would a better alternative?

Realm Stats page has the answer to the question you seek:
http://www.warcraftrealms.com/realmstats.php

As for finding what the Boston Datacenter has:
http://www.wowwiki.com/US_realm_list_by_datacenter
so go to wowwiki and find your realm, then come here and see its stats.

The BG data might be a lil more harder to find... but as always the other faction always wins more games no matter what realm or BG you are playing on.