Matt,
I like the idea and the noises generated very much.
One thing that comes up in my mind is thermal insulation. The tunnel area, for some reasons, transfers a significant amount of heat into the cabin. The exact cause is unknown to me, but I think it was from the convection from the engine bay and then was aggravated by the two exhaust pipes under the tunnel plate. With increased combined surface area of four primaries in each side, I suspect there would be a significant increase in heat transferred into the cabin.
By how much is ground clearance reduced by stacking the primary pipes in such fashion? Does the lowest point of the collectors still sit above the frame rails? Would a "tri-Y" design simplify and reduce the plumbing while keeping the same objective?