You'd need to back to back them, but I would say probably the one with the smallest volume to achieve 600bhp. And it doesn't aid spool up, it improves throttle response. This is because the TB is now only an inch or so from the cylinder, instead of the several of the single TB arrangement, so the time from the throttle opening to the time the air is delivered into the engine is pretty much instant (so same effect as throttle bodies on an NA engine).