As a warpriest counts their level as his/her BAB for any attacks made with their sacred weapon, does that mean they get multiple attacks at +6 BAB with their sacred weapon, or does that have to wait until they reach 8th level when they reach +6/+1 on their normal progression ?
The first one. When a warpriest wields his deity's favored weapon or any weapon that he has Weapon Focus for, he's treated as full BAB. So at 6th level he'd get a second attack, and so on.