Well, I have to say that using Bluetooth on my 8700g doesn't seem to affect battery life all that much (maybe 5%-10% difference on what's left at the end of a working day.)
For what it's worth (in case it makes any difference) I use a Jabra BT500 with it, and leave the 8700g in non-discoverable mode. I think the Jabra is quite intelligent at least conserving it's own power. I don't use the phone *all* that much, but I've only had the Jabra complain it's battery was low - after a week's use - on one occassion. My 8700g (with Bluetooth enabled) typically shows 80 or 85% full from full charge at 7am, to end of play at 7pm.