Hi Matt,
I'd suspect it's tricky to compare those two combustion chambers
side by side, coupled to the same heat exchanging set up. Mainly the batch rocket does need a wider channelled system because of its dependance on higher gas velocity. Every time a batch box comes by which doesn't work satisfactory there's something wrong in the department of cramped stream openings, too tight corners or the like. In order to keep the air speed high every bend seems to be 1.5 times as wide as system size. In at least two instances that solved sluggish behaviour.
No hard evidence, just a hunch that this firebox is better suited to a spaciously bell system in order to keep things going. Ultimately, the burn rate of a batch box is about twice as high as a comparable J-tube so those sytems aren't completely alike.
Looking forward to meet you at the innovators gathering. There's one new idea for both the Evans rocket and the batch box rocket on the back burner (pun intended) I'd love to try those out in Missoula.