I'd go this route. Try swapping out the old spring just to see how it feels. If it's like the pedal of old that you loved, loosen the spring tension until they feel the same.Iron Cobra pedals tend to have a heavy feel to them, it's just one of their "features."
It's possible that a brand new spring is making the new pedal feel even heavier. You could try reducing the spring tension a little more, or even try putting the older spring from the other pedal on the new one and see how that feels.