I already added dividers in each runner which actually to my surprise made it react worse....
"Tuned length" runners can add torque if their length is matched to the valve timing and rpm range of the engine, but their rpm range is limited and when they're out of this range they'll actively reduce torque. So what I'm saying is it isn't enough to have equal length runners, they have to be the right length. The fact that the runner configuration is less than ideal (no taper, no entry bell, no corner fillets, sharp change of direction) may mean that the flow loss with the dividers may eat up much of the gain from a tuned length anyway.