I just got my replacement 4.6" MnZn PC40 ferrite toroid. I'm finalizing my transformer design now, I'm going to maximize mutual inductance between primary and secondary, thereby maximizing the re-routing of their forces with the positive feedback loop. I'm also bifilar winding the chokes on the same core, I have worked out how to rephase them all to work in harmony...
Only thing I'm still deciding is whether to wrap the primary+secondary bifilar, or wrap the primary on top of the secondary. Basically bifilar would increase mutual induction, but primary on top of secondary would allow higher step-up with larger diameter primary wound on top of smaller diameter secondary.