I've been doing BG's with my 5x shadow priests since lvl 68 and hit 71 last night. Went straight into AV and was AMAZED at how much MORE xp you get (wish i had numbers). The honor and xp gain in the 71-80 bracket is SO much better. 2-3 hours later, I had a full half a lvl (which beats out northred collection quests).

Don't worry about doing BG's at 71 - there's lots of lowbies in AV. We won 80% of the AV games we played last night with all my priests at 71 - and both sides were full.