Ashley - just some thoughts, if you do a ftb render of everything , without btf render, (ignoring any problems with that) and with the culling , does it then makes it faster, and is there a speed difference in btf and ftb without culling, can you test the amount of fillrate change?
putting a couple of big rotating squares on each other should be a good test , did you test on a gpu limited pc
well i do hope this gets somewhat of a succes, if not now maybe on reviewing