The spring that goes from the clutch fork to the bell housing doesn't make the pedal stiffer, it returns the pedal to the top. That's why it's not some super stiff spring.
For a diaphragm clutch the overcenter spring on the pedal itself should be removed, because it can cause the pedal to stick at the floor when the pedal is fully depressed and there's no need for it with the action on a diaphragm clutch. And with a diaphragm clutch the feel is definitely different than with a borg&beck.