I do believe that the arena bow is pretty good for BMs, and if you really want it, you should aim for it. To be honest, the Sunfuy bow has a low (and i mean like 12-24% low) drop rate. I've been in kara for the past two months and i've yet to seen it drop.
If you do go for any weapon outside of pvp that's not in kara, i'd look at some of the ZA weapons in there. They compare to the sunfury bow pretty well.