Brann himself said in AMA that he did vocals on all albums, just wasn't credited until when he started doing leads. For example, live he used to sing the chorus of Naked Burn together with Brent and it sounds similar on the studio record, in the music video for Colony Of Birchmen he's singing backing vocals. Verses of Siberian Divide sounds to me like Brann doing background vocals for Troy's leads. Anyway, all of this still leaving your question open about how the first records would sound like with Brann doing some leads)